this is a known bug. We're currently fixing it (it's been reported as a support ticket).
But even when the bug is fixed, the "count" column for percentile won't show the same value as for normal aggregations, for two reasons:
a) It shows the number of "observations" that went into the percentile estimation algorithm, not the actual count
b) the resolution is always one hour.
I'll post here again once the bug is fixed. Best regards,