'Why is rowspan not working if i switch the order of the cells?

I have a really simple example that is driving me crazy. Can someone explain why is this working

<table>
  <tr>
    <td>January</td>
    <td rowspan="2">$100</td>
  </tr>
  <tr>
    <td>February</td>
  </tr>
</table>

While this isn't?

<table>
  <tr>
    <td>January</td>
  </tr>
  <tr>
    <td>February</td>
    <td rowspan="2">$100</td>
  </tr>
</table>

html layout html-table

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