Yes, i think its what you say.
There are prisoners included on the battle resolution engine, but they function better for the occasional surrender on an open field battle, where a defeated army can retreat, but don't work that right for cases of mass surrender of cornered forces.
As i understand the problem, in an assault, the attacker will surely suffer more casualties than the defender (specially if the fortification is not completely breached) up to the point were the heavily outnumbered defenders finally break and try to run... only to discover that the can't escape as they are inside a structure.
From then on, the defenders are completely broken and cornered and will be slaughtered to the last man instead of surrendering "en masse".
Now, on assault where the attacked is repelled before the defender break, you will notice how they sustain much more casualties.
Its something that could use some improvement in future games... someting like making cornered units that break auto surrender instead of being killed to the last man.
The result is the same in the end (the enemy is no more) but it will look better to see them as prisoners on the Battle report instead of normal casualties.
Cheers!