In this case, this is due to the nature of ancient Germanic personal names. They were “dithematic,” i.e. composed of two meaningful elements (though there were “hypocoristic” names, of which only one element was commonly used; over the course of time, these monothematic names became normal names of their own right—cf. Hugh/Chuco, Charles/Karl or Otto). This onomastic structure became very common in Europe as a result of the Germanic take-over of the majority of the former Roman West. These elements were transmitted within family groups and recomposed to form new names (here is an example [1] of the transmission of a few elements in a 7th-century Frankish family). The choice of these elements was precisely made to emphasise one's position within a family group. Over the course of time (in royal families at first, and then in the aristocracy), recomposition stopped and was replaced by transmission of names (something historians link with the transformation of family structure: belonging to a (paternal) line became more important). But these names were still inherited from the previous system; therefore, the plurality of Ed- names reflects the relative frequence of the theme “Ead” in Anglo-Saxon royal onomastics. Similarly, several continental names were formed with the component Adal- (A(da)lbert, Ad(al)olf), whose Old English equivalent was Æthel- (Æthelberht, Æthelbald—though its influence was not, in this case, paralleled in modern personal names).

[1] from Régine le Jan's Famille et pouvoir dans le monde Franc.