So when you have 1,420 slingers, 210 horse, 500 hops and 16 catapults as an offense, it just decides to kill all the catapults?
By the same logic, if I attacked with my 40 manticores, they'd die first?
yes it is a bit weird that it would kill ALL the 'pults, but there you have it... and yes, if you had manticores in the attack they would have an even higher chance of being hit

the thing is that it takes the % from your total attack and then allocates it randomly, since your 'pults (or mantis) occupy more slots they are more likely to be hit.
Then why didn't all the horses (or almost all) die as well? IN fact, in that attack, the only unit that seems to have been killed in 10-30% were the horses :S
not quite sure what you're saying here but i assume the bolded word is supposed to be catapults? i think you'll find that the horses would have also had a higher casualty rate than the slingers or hops, although horses are only 3x 'bigger' whereas 'pults are 15x.
slings: 29/1420 = ~2%
hops: 29/500 = ~6%
horses: 28/210 = ~13%
'pults: 16/16 = 100%
total FS killed: 382/2790 = ~13.7%
so overall it wasn't so bad, just unfortunate that the 'big' units took the brunt of it.