SSRS border questions

I have VB 2010 project and a using ssrs for reports.

I have a Matrix and have defined the borders of the text boxes.
some of them appear correct but some have a thick grey line in the centre and a thin black line above and below the line.

Also if find the borders on the edge of the table are thinner than the borders in the middle.

Any suggestions on what I am doing wrong.
Thanks.