Total Ascent Difference

This looks totally normal.  Different devices will variances in elevation data recorded as there are slight differences in each barometric sensor.   It is impossible to have them all manufactured identically.  When you use the elevation correction the elevation is calculated from terrain maps generated from satellite data.  These aren't perfect either and have errors in various spots.  All this means there will be different numbers from different sources.  I never worry too much about elevation readings.

It’s an estimate. Being within 10% is very close. 

Why you say it's impossible to have them all manufactured identically? The chipset is the same. For some people elevation readings are very important. The error that the device is showing is A LOT.

10% is not very close. And it's not 10% it's 13%. A LOT.

"Why you say it's impossible to have them all manufactured identically?"

It's not enough that they all produce the same number. If it was anything like as easy as you are implying it would already be done. 

It's an estimate. There is no one standard way of making this measurement.

"For some people elevation readings are very important."

Why is it "very" important?

It's a very approximate measurement of difficulty since it doesn't take grade into account.

The pressure sensor output is what the chipset uses to determine altitude.  Absolutely everything that is manufactured is slightly different due to manufacturing tolerances.  All these pressure sensors will produce slightly different outputs for the same input pressure.  These slight differences can produce what you are seeing.

All measurements have tolerances as well.

You could have more accurate elevation numbers but it would cost you a huge amount more.  Would it be worth it really?

Do you work for Garmin?

do you train?

Your wishes don't change how the world works.

If you train with any sort of understanding, you would know that gain is not a good measurement of effort.

Yes I train.  No I do not work for Garmin.  I do work in manufacturing so I understand tolerances.  I don't pay that much attention to the elevation readings from my Garmin.  I don't know how accurate they are.

So you work for Garmin and you don't train.