Here are some reasons why they are different -
Wordpress stats measure only page views. Google measures everything, so if you are looking at the first screen in GA vs WS, WS would be a lot more.
GA is cookie based, so if the visitor has cookies disabled you won't record that visitor.
(I don't remember which one, but one of them) don't count PDFs, word docs as pages… if you have them.
WS sometimes counts the bots as visitors, GA is much cleverer in this area.
If your site is slow, it will take time for google analytics to load, so if the visitor leaves before the GA code loads, it won't be recorded but WS would have recorded it.
Pick one, GA is much much reliable so if it were me I would pick that, and stick with it.