At the moment in shiboken, the isConvertible methods from the primitive types handling seems not to be accurate.

For example:

• For Integers, we use the function PyFloat_Check
• For Floats, we use the function PyInt_Check(pyIn) || PyLong_Check(pyIn)
• and the same happens with long, bool, etc.

Also, the complementary methods isOtherConvertible is mainly using SbkNumber_Check which is an alias to PyNumber_Check, function that's too permissive since accept both integers and floats.
On CPython's HISTORY file (Misc/HISTORY), there is a statement that suggest its deprecation in the future:

PyNumber_Check() is not very useful to determine numeric behavior.
  It may be deprecated.)

(primitive types code)