float（3.90-3）没有给出3的答案

Question

Python 3.7.3 (v3.7.3:ef4ec6ed12, Mar 25 2019, 22:22:05) [MSC v.1916 64 bit (AMD64)] on win32
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>>> float(3.90-3)
0.8999999999999999
>>> 

预期答案为3。为什么python回答0.8999999999？