Fix area_ratio calculation for uniform distribution

[maxime-vaillant]

I was facing some issues using DropEventByArea and found this mistakes.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 76.50%. Comparing base (a4e8a45) to head (c5c1bf3).
⚠️ Report is 136 commits behind head on develop.

Files with missing lines	Patch %	Lines
tonic/functional/drop_event.py	0.00%	1 Missing ⚠️
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Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #318      +/-   ##
===========================================
- Coverage    77.07%   76.50%   -0.58%     
===========================================
  Files           54       48       -6     
  Lines         3067     2992      -75     
===========================================
- Hits          2364     2289      -75     
  Misses         703      703              

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