The DatePicker control contains a ‘gap’ of a couple of pixels on the left:

You can’t really see it normally, ‘cos the blue of the gap matches the blue of the area to it’s right – but you can see it in the borders of the heading row for the days of the week. It’s a real problem – looking in the code it’s just a normal table cell, with no special style applied. However, there are lots of table cells in that table, so it’s difficult to pick it up individually. And in other styles it becomes more obvious.

I don’t really have an answer, other than making sure that your styles match colours – and it’s not going to be possible to fix that header…