CSS and Table/Tr row sizing

I created a table and I have the following problem. I have a row with Table Headings as the first line. The size of the second row and I think the size of the letters in the second row is different than in the third row.

<table>
 <tr>
  <th>1</th>
  <th>2</th>
  <th>3</th>
 </tr>
<tr>
  <th>510px x 45px</th> *Letters also seem to be bigger
  <th&开发者_如何学编程gt;510px x 45px</th>
  <th>510px x 45px</th>
 </tr>
<tr>
  <th>510px x 37px</th>
  <th>510px x 37px</th>
  <th>510px x 37px</th>
 </tr>
</table>

Attached:

• Picture showing ROW 2

• Picture showing ROW 3

I am sure that I have exactly the same code for the two rows because I copy / paste Row 3 code to Row 2 but the problem persists.

Can someone help me?

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was clearly different, because obviously in the second row you write 510px x 45px, whereas in the third row you write 510px x 37px..

(510px x 45px) <> (510px x 37px)