Problem is your data contains NaN values, so int is automatically cast to float.
I think you can check NA type promotions:
When introducing NAs into an existing Series or DataFrame via reindex or some other means, boolean and integer types will be promoted to a different dtype in order to store the NAs. These are summarized by this table:
Typeclass Promotion dtype for storing NAs
floating no change
object no change
integer cast to float64
boolean cast to object
While this may seem like a heavy trade-off, in practice I have found very few cases where this is an issue in practice. Some explanation for the motivation here in the next section.