Garmin have different opinions about how high my MaxHR is!?

It's not an error. In the heart rate graph of the day you see the 2 minutes average between 14:28 and 14:3030, so this average 180  is lower of you max heart rate that you reached at the moment 21:50 in the run.

You shouldn't worry about it. If you have "max heart rate autodetect" in the settings, then the watch should have updated your max heart rate (but maybe only if you had a chest strap, I ask opinion of expert about this)

Mirko's pretty on to it. The single spike is usually ignored when the watch looks to adjust your max heart rate as it looks for an average over a period of time as means of ignoring those spikes.

This seems awfully silly as max heart rate isn't something that is usually sustained but a number that is only touched and not exceeded?
Regardless, in the Garmin Connect App They report "Daily Max Heart Rate" and that doesn't actually incorporate your workouts "Max heart rate" because they're ignoring anything less than 2 minute max's.  Just recently I had a sport maximum of 196 that same day only claimed a maximum of 186..  The day calculation should at least take into account your sports maximums because they are more accurate than daily readings, since sports polling interval will be far more often than daily and typically higher than overall day max.

This is troublesome in the case that for example I want to track my max heart rate over time: I would look at Garmin's weekly and monthly totals for heart rate and they will not incorporate true maxes from workouts where the max is typically encoutered.

No, the daily max heart rate (that is the average heart rate of two minutes) is not used for heart rate zones. To calculate heart rate zones and your real max heart rate, if you have "autodetection on" in your settings the watch considers the max heart rate that you have in one second in one of your activity. Before using that value, it asks if you want to accept it. It then appears in report in the graph of "max heart rate" . 

The daily max heart rate (two minutes average) is for healt statistics.

The max heart rate during an activity (value of 1 second) is for sport statistic.

Abberrant spikes need to be ignored otherwise they will skew the data. It makes sense for Garmin to look at the max heart rate over one or two seconds rather than a single beat. In one or two seconds you would expect more than one beat at max heart rate.