Lines not lining up flush... Do you know why?

Hello,

For some reason my lines are not lining up flush in particular areas, but then good in others.

I'm creating very basic drawings of simple mold layouts on a platen. I start with my main area of the platen then begin laying in the molds; this is all squares and rectangles, nothing flashy. By mouse icon will show the lines are flush, but then as I zoom in and out there will suddenly be a very small gap between edges... I can see it because as I move around it will suddenly be bold... This is very annoying and just messes with the overall dimensions.

See images for a detail of the bold lines and a zoomed in view of the gap between edges. 

Do you know how to fix this and why it is occurring? What do I need to do to fix this?I want to avoid having to zoom all the way in just to avoid this. They should line up when the mouse indicates they are flush...

Any links that may cover this topic would help as well.

Thanks!