Detection of OpenPGP data is not robust

OpenPGP messages are a container for arbitrary data, as such, I expected diffoscope to descend into it and compare the contents, but that is not what I observe. Consider:

% diffoscope a.tar b.tar
--- a.tar
+++ b.tar
├── file list
│ @@ -1 +1 @@
│ --rw-r--r--   0 teythoon  (1000) teythoon  (1000)        0 2020-08-17 16:21:02.000000 foobar
│ +-rw-r--r-x   0 teythoon  (1000) teythoon  (1000)        0 2020-08-17 16:21:02.000000 foobar

But no dice when inspecting these tarballs inside signed (and compressed) OpenPGP messages:

% diffoscope a.tar.pgp b.tar.pgp
--- a.tar.pgp
+++ b.tar.pgp
@@ -1,31 +1,31 @@
 00000000: a301 9bc0 cbcc c0c1 d871 a7ce d822 be52  .........q...".R
-00000010: 9671 ad06 7712 6ba2 5e49 6251 bcd5 ba2d  .q..w.k.^IbQ...-
-00000020: 69f9 f949 8945 0c34 0506 4060 6662 02a2  i..I.E.4..@`fb..
+00000010: 9671 ad06 7712 6b92 5e49 6251 bcd5 baa3  .q..w.k.^IbQ....
+00000020: 69f9 f949 8945 0c34 0506 4060 6662 0aa2  i..I.E.4..@`fb..
 00000030: 0dcd 4d0d 9069 1860 3034 3637 3433 3532  ..M..i.`04674352
-00000040: 3734 3563 3030 3432 3533 6050 30a0 adb3  745c004253`P0...
+00000040: 3734 3563 3030 3432 3533 6450 30a0 adb3  745c004253dP0...
 00000050: 20a0 b418 1812 0a0a 0c25 a995 2519 f9f9   ........%..%...
 00000060: 79b8 d411 921f 05a3 6014 8c82 5130 0a46  y.......`...Q0.F
 00000070: c128 1805 a360 148c 8251 300a 46c1 2818  .(...`...Q0.F.(.
 00000080: 05a3 6014 8c82 5130 0a46 c128 1805 a360  ..`...Q0.F.(...`
 00000090: 148c 8251 300a 46c1 2818 05a3 6014 8c82  ...Q0.F.(...`...
 000000a0: 5130 0ae8 083a 198d 5918 1839 1864 c514  Q0...:..Y..9.d..
 000000b0: 5954 b3fc 429f 2cd7 9dbe 5625 e379 c79d  YT..B.,...V%.y..
-000000c0: 3a63 8bf8 4a59 56a6 78ab 755b 18b8 3805  :c..JYV.x.u[..8.
-000000d0: 6022 a5fc ecff 23e6 7f4d 9b1f cc95 a77b  `"....#..M.....{
-000000e0: a741 77c3 9aeb cc96 f2bd df73 762d fdaa  .Aw........sv-..
-000000f0: 912c f12d 82f7 e8a1 cefa 4391 b566 3bf4  .,.-......C..f;.
-00000100: 446f 6e36 daea c5f7 f981 e6a5 d57b 7d8c  Don6.........{}.
-00000110: fe9c 9e14 1621 b65d 68ad 8ce5 8e44 0b39  .....!.]h....D.9
-00000120: 7ddf f3a7 f5e2 266e 76d6 3268 3b7e fa67  }.....&nv.2h;~.g
-00000130: eac4 5dc1 3932 2553 a72f 3acd 5db0 b4b6  ..].92%S./:.]...
-00000140: 3389 41bc 53f0 c56b 9f05 aeb9 f516 de02  3.A.S..k........
-00000150: 9dcb 5ac5 7438 32d5 7d1b 1ab7 3695 3e5a  ..Z.t82.}...6.>Z
-00000160: 6560 3149 71a3 32a3 87ed 8a2e 8dbb 5e2c  e`1Iq.2.......^,
-00000170: ef65 1f35 76fe b831 6f81 cf15 ee19 2f1b  .e.5v..1o...../.
-00000180: 4556 a7b4 1a32 3f35 60b8 2273 c1e5 ddf9  EV...2?5`."s....
-00000190: 7fab be36 c872 f05a 6578 cfbd 75e5 f963  ...6.r.Zex..u..c
-000001a0: fd88 be93 5e81 53f6 6dd8 5bd6 c55f ebaa  ....^.S.m.[.._..
-000001b0: bf46 6a55 98f5 b9fe 597b a6ec 4ff8 14b4  .FjU....Y{..O...
-000001c0: e6d9 8592 b0be 9cc4 6da7 1e29 dfee 7d71  ........m..)..}q
-000001d0: ffdc f4bb 4f2c 7ef0 a6ef 7c50 2e21 f148  ....O,~...|P.!.H
-000001e0: 6351 6b05 00                             cQk..
+000000c0: 3a63 8bf8 4a59 56a6 78ab 7547 19b8 3805  :c..JYV.x.uG..8.
+000000d0: 6022 f75a 3818 666d fca4 cf1d a6b4 f7d7  `".Z8.fm........
+000000e0: 1e6d ebe3 fb7b bf5e 6029 db17 d8cd 3c5b  .m...{.^`)....<[
+000000f0: d35a 71f5 e923 5357 f02f f9ac 326d cb0f  .Zq..#SW./..2m..
+00000100: 65eb f57f 1ccc 665f 2e17 58d4 ff83 5d89  e.....f_..X...].
+00000110: 755e 88ef c43e fb1c 9d1b d1e6 460a 66a9  u^...>......F.f.
+00000120: 5752 ce99 5bde 72b8 7ab9 3ea0 f166 d455  WR..[.r.z.>..f.U
+00000130: dbc6 57b7 3539 fc26 4d59 79a7 af60 efad  ..W.59.&MYy..`..
+00000140: 2617 6537 0976 d68a 8349 7fca eeb7 7f9f  &.e7.v...I......
+00000150: f297 e7a3 7b75 eec9 d93f 8faf 3460 6c9c  ....{u...?..4`l.
+00000160: a9fe 6b7a 54b1 a5ea e116 1e1f a1e3 02ab  ..kzT...........
+00000170: 4fce 3fee fa94 f5f5 548d e36d 452f d374  O.?.....T..mE/.t
+00000180: fc77 e86f 918c eb16 8d9a 79e4 6869 8671  .w.o......y.hi.q
+00000190: 6d88 e132 973c 7b3d 8543 61ac 9d4e a257  m..2.<{=.Ca..N.W
+000001a0: 4f8b ed59 f0ee e5af e683 f78e fcf0 ac4a  O..Y...........J
+000001b0: 3d5d a7cc 30e3 3ef3 96a5 8b53 b927 eb73  =]..0.>....S.'.s
+000001c0: 4f9c 5a6a 7923 73f7 af76 f6fb 3bc2 1c4e  O.Zjy#s..v..;..N
+000001d0: 2cda e578 76fb 9497 b117 16b8 4faf 38ab  ,..xv.......O.8.
+000001e0: 7dcf fb81 3c00                           }...<.

a.tar

a.tar.pgp

b.tar

b.tar.pgp