this 17 days gap is negligible! since this is a minute period in terms of "alignment" with galactic center as lights from different parts of galaxy travels on different intervals and by the time we measure the location of entire solar system which is again moving on a faster rate (red color...
ok fine ! Earth is farthest in july and not in december but again it has a co-relation with galactical gravitational force ! as Earth aligns itself with the center of galaxy and sun twice in July and December the farthest and almost nearest and aligns with Solar system's locus/path in April...
earth's orbit is elliptical because in December when Earth is farthest from sun it is attracted towards galaxy for which Sun & entire solar system itsef has an orbit !