Mine (the charger, not the motorcycle jump-charger) has constant current and constant voltage phases, and gases quite a lot towards the end, before it goes to float voltage. I'd guess this is a deliberate "balancing" over-charge.
Its got three initial charging levels, with scooter, motorcycle and car idiot logos/lights.
What I usually do is run it to float three times, first at car, then motorcycle, and finally moped levels. Never been sure this was optimal but the above discussion seems to suggest it might be.