WSPR clock accuracy

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From WSPR 3.0 User’s Guide we know that computer clock must be correct to within about ±1 second.
I've tried to test it and find practicullar limits of error.
First PC is receiver, and have internet time sinchronisation.
Second PC is tranceiver and I setup clock as I want.

We can compare DT parameter on first PC, and get or get not spots from other WSPR world stations.
Band is 40M. Power is 0.5 W.

Accuracy clock manually setup is near one second.

TRX time________DT RX, c____World_spots
5 sec fast______no_decode____No
4 sec fast______no_decode____Yes
3 sec fast______-2.2_________Yes

win xp
(~0.5 sec fast)_-1.6_________N/D

same, manually__-1.3_________N/D
2 sec slow______+1___________N/D
4 sec slow______+1___________Yes
6 sec slow______+3.1_________Yes
8 sec slow______+5.6_________No
10 sec slow_____+6.5_________No
11 sec slow_____+6.5_________No
12 sec slow_____no_Decode____No

Internet clock sinchronisation on Windows XP have accuracy near 0.5 sec! It is no good, but WSPR can work with such accuracy.
So We can have 3 seconds accuracy when WSPR 2.12 K1JT can work!