Yes, whenever i get 6 tags, all of them have the same timestamp.
Old hypothesis:
Notation: System = 3DS = Person. Hotspot = Server = StreetPass Relay Point = Nintendo Zone location.
a) Each Streetpass Relay hotspot works independently. A hotspot keeps in memory the last 6 systems that went there (and spent time enough for the exchanging information, with wireless on, system at the Home Menu or sleeping but not in DS mode, and has all least one application with StreetPass activated).
b) For each system, it keeps information for each game the system is Streetpassing. So, among the 6 systems, maybe only 4 are using Mii Plaza, only 2 are using Pokemon X, only 1 is using Super Mario 3D Land, etc. (So if you don't have all possible games, you don't get the full information from the 6 systems. Depending on your applications, you may miss one system in its entirety.)
c) When a new system makes contact with the server, it becomes one of the 6 memorized systems, and the whole data of the oldest one is discarded.
d) Even if one system is on saved on the server, one won't receive any data if s/he has Streetpassed the same system in the last 8 hours (either normally or by relay points).
I did an experiment today:
I went to 3 McDonalds today with my girlfriend (each one own a 3DS). Each McDonalds was a Nintendo Zone. The whole trip was around 2 hours, so we shouldn't have gotten any repeated people. We only checked the tags for Mii Plaza. I have strictly more Streetpass applications than she does.
1) For the first, she got their tags some minutes before me, and each of us got 5 tags. (Agrees with my hypothesis.)
2) I had two times for the same batch of Streetpass Relay tags. Something like 2 at 13:45 and 3 at 13:46. I think it may happen if you exchange data very close to the moment the minute changes.
3) For the second, she got them 1 minute before me, but she got 6 and I only 3. (THIS ONE HAS NO EXPLANATION AT ALL.)
4) (We made sure to open the Mii Plaza so neither of us would reach the ceiling of 10 tags.) For some reason, I only got 7 people out of the 8 tags of the first and second hotspots, and one of them was my GF's system.
5) For the third, each of us had the same time for the tags, so I don't who got the tags first. But she got only 3, and I got only 2. (This would be possible if few people were Streetpassing Mii Plaza because they use other games, or if they are the same from the previous Nintendo Zones. Unlikely, but still fit in with the hypothesis.)
New hypothesis:
??????????