Vivado Design Suite Reed-Solomon Decoder
$\left\lfloor\frac{n-k}2\right\rfloor$ is the maximum number of bit errors that we can guarantee a code can correct for a binary channel.
But that doesn't mean a code can't sometimes correct more! (In general: not all code words have a distance that's equal to the code's minimum distance. It is the case for minimum distance separable codes, like RS codes are, though.)
Then:
RS codes are also used in applications where erasures, not bit flips, happen. You'll find the decoder documentation contains a section on erasure decoding.
You'll also find that a RS code can correct up to $n-k$ erasures – and my guess is that they made the count of that and the count of corrected errors the same width for design consistency reasons, and maybe to make different decoder IP interchangeable.
Note that giving a unsigned number more bits than it needs has no functional or FPGA utilization downside in IP cores: if whatever takes the output uses less bits than it has, the synthesizer will optimize away all calculations resulting in the bits that weren't used.