title: Funny (rounding?) errors when adding
body: <p>One column has numbers (always with 2 decimals, some are computed but all multiplications and divisions rounded to 2 decimals), the other is cumulative. The cumulative column has formula <code>=&lt;above cell&gt;+&lt;left cell&gt;</code>.</p>
  
  <p>In the cumulative column the result is 58.78, the next number in the first column is -58.78. Because of different formatting for zero than for positive or negative numbers, I spotted something was wrong. Changing the format to several decimals, the numbers appear as:</p>
  
  <pre><code>                                    £58.780000000000000000000000000000
  -£58.780000000000000000000000000000  £0.000000000000007105427357601000
  </code></pre>
  
  <p>The non-zero zero is about 2^(-47). Another time the numbers in the same situation are:</p>
  
  <pre><code>                                    £50.520000000000000000000000000000
  -£50.520000000000000000000000000000 -£0.000000000000007105427357601000
  </code></pre>
  
  <p>How can that happen?</p>
  
  <p>Also, if I change the cell in cumulative column into the actual number 58.78, the result suddenly becomes zero.</p>