If you read Bede's "On the Calculation of Time", actually written about 720 anno domini (plenty of translations from the latin), you'll find that Hreðmonað, roughly our March, was the month of the Goddess Hreðe. This was followed by Eostremonað.
Basically, the pre-Christian Anglo-Saxons used a solar/lunar calendar and calculated it so that the first day in Eostre's month (Eostremonað) was when the first wisp of the moon showed following the equinox. Thus it will always be later than the vernal equinox in the same way that Easter is.
Bede explains it quite well in his treatise (qv) and since it is the only explanation available, I feel we ought to honour it.
;-) Ian
btw: having lunar months in a solar year has the interesting effect that no corrections are needed to the calendar ... every now and then they just threw in an extra month in the summer. Fixed.