The voltage difference is because the chemical reaction for each type of cell is different, resulting in a different voltage. Each individual cell then gets connected in series &/or parallel to make up the required voltage & capacity however the physical size of the packaging imposes limits on just how many cells can be used.
NiMH do have a memory effect, but it's not as bad as NiCad, There are also limits to the number of recharges the different types have.