A deviation of 0.0015" can easily be the result of less than ideal toolpaths and finishing strategies. Aside from which, without "tweaking", getting better than 0.002" precision is as much a matter of luck as anything else. Thermal expansion can account for such an error, as can machine flex, tool flex, backlash, friction/stiction, under/over-sized tools, and a host of other issues. Bottom line - what you're seeing is about what you should expect, unless you're willing to iteratively "dial in" each part.
Regards,
Ray L.