Most active healing abilities don't heal to full health.
USF didn't either until they made the ambulance a toggle rather than an active ability. (Why did this happen btw? To cater to scrubs who couldn't micro it?)
Brit heal and ostheer medkits work the same way, they heal about 75-80% so if you're extremely low on hp you won't get quite to full health. Maybe there's some variance in the exact numbers but this has been my basic experience with such abilities.
IIRC it was pitched as an anti-blobbing thing. Since it's not a timed ability, no need to bunch up all your units at once to get it. Since you're not bunching up your units all at once, you're less likely to wander into the field in a massed group when you're done.
That said, ambo micro was dumb. It took up apm for no reason in a faction that was already overflowing with apm sinks.