Just like some numbers are periodic in decimal (1/3), some numbers are periodic in binary. 1.15 is one of them.

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1.15 (b10) = 1.0010010 (b2)
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It can't be represented exactly as a float without infinite storage.

>perl -e"printf qq{%0.16e\n}, 1.15"
1.1499999999999999e+000
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Previously mentioned sprintf "works" because it rounds instead of truncating.