In Einstein’s Book at pages 98 and 99 he wrote:
I.     That the whole gravity field of the universe is determined by the matter of the universe;
II.    That the universe is infinite (he refers to the boundary conditions at infinity);
III.   The relative velocity of the stars is small compared to the velocity of light (c);
IV.   Co-ordinates can be selected such that the whole gravity of the universe, where there
       is matter, is nearly constant;
V.    The universe for which he wrote his equations is an imagined universe in which there are
       no stars or planets but in which there is a continuous distribution of matter; in other words,
       everywhere in the universe matter is identical and equally and evenly distributed;
VI.   The universe is of constant curvature, either spherical or elliptical (ie. a mathematically
       perfect model) and is bounded (ie. finite).

Having changed his friend Grossmann’s equations, and having written equations for an imagined universe which is very different from the real universe, the question is, why did Einstein’s equations provide a better model of Mercury’s orbit than did Newton’s equations ?  The answer is that Einstein wrote his equations to give the correct answer for Mercury's orbit, which is one of the reasons they don't provide the correct answers for the universe in general.