Looking at it again, I see you are right, 5th level 1/2 casters do not have enough spell points to cast all of their spell slots. However, the problem is not with Roll20, it is with the DMG. "For a paladin or ranger, halve the character's level in that class and then consult the table." (DMG p288) By rule, all fractions are rounded down unless specified otherwise. Since they didn't specify it, that is the problem. Roll20 is technically correct, even though we all know in this case it should be rounded up to give the proper number of spell points.