being taught that it fluctuated somewhat
As indeed it does. But for purposes of calculating Easter, the Christians normalized it to a specific, more or less correct, date. The date of the full moon varies widely, but the timing from the new moon is (also more or less) farily consistent and it is this timing from the new moon that was normalized. My understanding included the datum that the dating of Pesach was counted from the actual equinox which can differ slightly from the Christian normalized one. If the full moon comes between the actual equinox and the Christian normalized one the dates of Easter and Pesach can be close to a month apart.
