While I don't have a specific explanation, I can say I have seen this behavior from time to time w/ Windows. I suspect it’s either a driver problem, or maybe a system tray application (esp. if it’s network related), that is waking and attempting to access the network before the wireless adapter has reestablished itself. But in all honesty, that’s just a guess. But I would at least try shutting down running applications, one at a time, BEFORE entering sleep state and see if it improves. You might be able to isolate the offender.
If the problem persists, one way to circumvent the problem is to use a wireless ethernet bridge. Because the bridge always remains active, even when the PC is sleeping, network recovery is INSTANTANEOUS. Just another one of the many reasons I prefer wireless ethernet bridges over wireless adapters (another reason? a wireless ethernet bridge preserves WOL (Wake On LAN)
[wireless router]<-- wireless -->[wireless ethernet bridge]<-- wire -->[pc]