Multiplying float number in XSLT

I want to multiply float number in a node with 1000 and the result I have to map to target node. Here is the XSLT code I have tried.

mnLineNumber * 1000

Here mnLineNumber node is float data type and it has value 1.0005. I have expected the result as 1005. But, I am getting the result as

1004.9999999999999.

Then, I have used the round function and I got the expected result.

round(ns0:mnLineNumber * 1000.0).

But, my question is why I am getting a floating number if i am not using round function in xslt.

Thanks,