As a parent of kids who were born on thanksgiving, and good friends with a mom whose daughter's birthday almost always falls on Memorial day weekend (and has had more than one party where only 1 friend was able to attend), I can say that there are some practical reasons for having a b-day party that isn't at the same time of year as the actual b-day - any time the b-day falls around a long weekend or winter break or is in the thick of a holiday or busy-time-of-school year (e.g. near graduations, mothers day/fathers day, etc.) the family finds that either the majority of the friends will be out of town, they have too many other things going on to add "party planning" to the list, or both.
And for a January b-day, given that many families are already 'partied out' with winter break and december holidays, it's not surprising that a family might prefer to have the party in the summer time esp. if all their prior b-day celebrations have been indoors in cold/rainy weather.
Just my $0.02. We usually hold our daughters' b-day party in late October because we can have it outdoors in the back yard with less worry about a rain out :-)