Laptops run on "burst" computing profiles in a lot of engineering situations, occasionally this applies to both the thermal design (runaway heatsoak if used at full tilt) but also battery design. I've seen several machines that will dip into their battery in addition to the charger to boost performance and dump wattage into the chips beyond what would be available from the adapter alone. I don't necessarily think it's good design, but modern battery chems don't really give a shit about up/down momentary charge cycles. Also the Chem they're using is not LiPo based as that chemistry while allowing for significant amperage to be drawn, is not stable enough for a laptop that is generally expected not to ignite.