From December 2015, it is possible to have non-integer conversion numbers ("fractional conversions") at AdWords. This is because some attribution models attribute fractional credit for each conversion across multiple clicks.
These attribution models determine how much credit each click gets for your conversions. You can attribute the credit to the customer's first click, the last click, or a combination of multiple clicks.
Even if you don't use an attribution model that gives fractional credit, you'll still see 2 decimal places in your reports.