Why does Orwell use the word "unaccountably" about Squealer's absence from the fighting?

I assume that you are referring to the part in Chapter 8
where Squealer is said to have been absent from the fighting in the Battle of the
Windmill.  If so, I would say that Orwell is being sarcastic.  He is playing up the idea
that we really should not find it at all surprising that Squealer was not involved in
the fighting.

If you look at the account of the fighting,
the pigs are pretty much absent.  We hear about cows and sheep and horses fighting, but
not pigs.  We should realize by now that the pigs are really just using all the other
animals -- the pigs rule and have everything easy while everyone else works hard (or
risks their life) even though their efforts end up benefiting the pigs more than anyone
else.

So I think that Orwell is being sarcastic -- it's
like "Oh, I CAN'T BELIEVE Squealer wasn't there!  I'm SOOO surprised... I CAN'T account
for it..."