Webster 1913's 'exactly suitable; fit; convenient; timely' has, over the intervening ninety years, come to be more commonly used in a derogatory sense. Nowadays, if something is described as being 'pat', it usually means that it is
superficial,
too good to be true, or suspiciously convenient.
This undoubtedly comes from the common 'X is too pat', as in the phrase "Obi-Wan's assertation that Luke's father was dead "from a certain point of view" seemed too pat for my liking; I think that George Lucas is making this up as he goes along".