First, check the battery in the meter to make sure it's readings are corect.
If it's ok, and they are, then:
The PSU isn't likely designed with current limiting of the type a charger would have, so it's just maxing out the current it can output, which will heat it up more than it's labelled limit would. If it's overdesigned that could be ok, but if it's not it could overheat parts and cause failure (or even fire).
A regular PSU doesn't necessarily limit current at all, and may just try real hard to fulfill any demand placed on it, until parts burn out. Usually voltage drops significnatly as load increases, and some have shutdown protections if that goes too far down, but some don't. Sometimes you can see quite high currents out of one until something explodes.
A charger, or a constant-current PSU (like LED PSUs, like the Meanwell HLG), will only go up to whatever it's current limit is, and then it will automatically drop voltage to the point current stays below that limit.