pfft~ frame sync

I have a pfft~ sub-patch instanced with:
pfft~ bocaFFT~ 1024 2

in bocaFFT~ i am doing
fft ~ 1 hanning

I am using the third outlet (fft bin index)
to generate a frame sync. I run the bin index
signal into >=~ object and then into an
edge~ to convert from a signal to a bang
when the fft index is greater than or = 511.

with a 1.6sec 48k audio file i get 77frame counted
in the pfft~ as opposed to the 46.8 frames i expected

i have been assuming that if my signal vector size is
1024 i could expect 48k/1024 = 46.8 frames per second

anyone have any ideas how i could account for the 77 frames counted
in the sub-patch and how it relates to the fft frame time??

m