reftex-search-unrecursed-path-first

reftex-search-unrecursed-path-first is a variable defined in `reftex-vars.el'.
Its value is

t

Documentation:
Non-nil means, search all specified directories before trying recursion.
Thus, in a path ".//:/tex/", search first "./", then "/tex/" and then
all subdirectories of "./". If this option is nil, the subdirectories of
"./" are searched before "/tex/". This is mainly for speed - most of the
time the recursive path is for the system files and not for the user files.
Set this to nil if the default makes RefTeX finding files with equal names
in wrong sequence.

You can customize this variable.