Ok,
Our tier promotion system is rather in-depth and takes into account quite a few different factors when determining a players tier placement. Tier is not entirely based on your average score. There are some other calculations involved that when combined together make up your tier. Part of that calculation is a weighting on your most recent scores that is heavily considered. The way the tier system works it will usually promote most players within 1 or 2 strokes of the same average, but that's assuming that most players are playing on a regular basis.
In the case of players who do not play stroke play on a regular basis (like the player you linked) they do not get weighted quite as heavily since they have scores spread out over many weeks and may not have any "recent" scores. Players who only play once every while will usually take just a little longer to reach the promotion level (slightly lower #). If that player was to come back and play 2 or 3 rounds back to back he would likely be immediately promoted to the Pro Tier since he would then have a lot of "recent" scores to weight.
Basically, depending on the total number of stroke games you play the point at which you cross the promotion line will vary. Since the player you linked played a game about once a month or so, he would probably be one of the most extreme examples of this. Most active Amateurs will turn pro in the 77-78 range, give or take.
As I mentioned above, there are even more calculations involved than what I'm able to discuss here. We keep part of it secret so players do not figure out how to "ride the line" and sandbag prizes in a lower tier, but I hope this helps you to understand why you see some players with lower averages than most in a particular tier.