Nowadays with computer typesetting, the score is done first, and in the part extraction, the parts will theoretically come out exact. Although this doesn't always happen, and many typesetters don't proofread adequately, it is generally far more accurate in terms of slurs and articulations than when everything was hand engraved.
In the old days (which wasn't that long ago, by the way - I think hand engraving really only finally stopped in the 1980s), It could be that the engraver was working fast and left some things out, and it could also be that an editor (who would possibly be a cellist, for example) would edit the solo part but the engraver would not necessarily transfer his changes into the full score as well.
Finding differences are a good chance for us to use our brains and our training to make a scholarly decision as to which is more correct/feasible/better. Sometimes you will decide the score is better, and sometimes the solo part.