The main reason for a probationary period for a 'new hire' is to determine whether the new person fits in with the organization or not.   When I hired, I always assumed the person could do the job; otherwise why would I hire them?  If they couldn't do the job, they would soon be gone.   The same is true of being the new person being hired.

After that, personality, work style, and work ethic are always important.  If the organization is a 'nose to the grindstone' 'get the work done no matter what' organization, the 'laid back' person is not going to work out even if the measureable results at the end are identical.  The reverse is true, as well. 

I have seen turnover that happened just because of those issues.  The election of J. Pat Horton as DA back in the 70's resulted in the replacement of about half the existing staff here in the Lane County DA's office in about 6 months.   They were all competent but he wanted a particular style in his office and he got it.