Why Is Google Port Scanning Me?

I’m used to the normal ‘Internet background radiation’ of hackers/bots scanning the common ports on my router (SSH, VNC, etc.) but recently I’ve noticed scans of multiple ports from IPs registered to Google.

e.g., I'm getting a few thousand a day of these:

[INFO] Sat Aug 22 11:43:44 2009 Blocked incoming TCP packet from 66.102.7.191:80 to xxx:47414 as ACK received but there is no active connection
[INFO] Sat Aug 22 11:43:44 2009 Blocked incoming TCP packet from 66.102.7.191:80 to xxx:15370 as ACK received but there is no active connection
[INFO] Sat Aug 22 11:43:44 2009 Blocked incoming TCP packet from 66.102.7.191:80 to xxx:63879 as ACK received but there is no active connection
[INFO] Sat Aug 22 11:43:44 2009 Blocked incoming TCP packet from 66.102.7.191:80 to xxx:7748 as ACK received but there is no active connection

Looking back I see these requests have been coming for months but have increased in frequency recently.

They come from a range of IPs which whois reports are assigned to Google:

66.102.7.101

66.102.7.191

74.125.15.22

74.125.15.93

74.125.15.100

74.125.15.157

74.125.19.118

74.125.103.33

74.125.103.96

74.125.103.97

etc.

So for what purpose is Google port scanning me?

UPDATE:

These are likely just delayed responses to a web page request made by your browser just before you quit your web browser.

http://forums.macosxhints.com/showthread.php?t=66719