Why does not the time difference between the two schedules meet expectations?

[suncle1993]

when I run the following code, I find the result is confusing me. Can anyone help me?

#!/usr/bin/env python
# -*- coding: utf-8 -*-

import sys

reload(sys)
sys.setdefaultencoding('utf-8')
import time
import datetime
import schedule


def work():
    print 'start: ', datetime.datetime.now()
    time.sleep(15)
    print 'end  : ', datetime.datetime.now()


def main():
    schedule.every(1).minute.do(work)
    while True:
        schedule.run_pending()
        time.sleep(0.01)

main()

The result is as follows:

start:  2017-11-09 14:16:32.320981
end  :  2017-11-09 14:16:47.336356
start:  2017-11-09 14:17:47.340923
end  :  2017-11-09 14:18:02.355287
start:  2017-11-09 14:19:02.361218
end  :  2017-11-09 14:19:17.375502
start:  2017-11-09 14:20:17.377436
end  :  2017-11-09 14:20:32.392757
start:  2017-11-09 14:21:32.401274
end  :  2017-11-09 14:21:47.415794

Why the time difference between two 'start' is not 1 min, but 75s?
Program execution time should be regarded inner time between the scheduling period.
in my viewpoint, the correct time difference is 1 min. the delay caused by time.sleep(15) should not affect.