Why the nonsense number appears?

[Johnny Lee]

Hi,
   Pls take a look at this code:

----------
>>> t1 = "1130748744"
>>> t2 = "461"
>>> t3 = "1130748744"
>>> t4 = "500"
>>> time1 = t1+"."+t2
>>> time2 = t3+"."+t4
>>> print time1, time2
1130748744.461 1130748744.500
>>> float(time2) - float(time1)
0.039000034332275391
>>>

   Why are there so many nonsense tails? thanks for your help.

Regards, 
Johnny