I am sure this has been asked, but I could not find it with the search function. This isn’t the only instance of this happening but last night I had a workout where my head unit said I earned a gold breakthrough, and I had filled or overfilled all of my magic buckets. However once uploaded I had only gotten a silver breakthrough and was low on my High and Peak buckets. I had just happened to take a photo of my computer last night because I had finished a 3 hour ride in about 70 min and was sharing with my buddies.
Same issue as with the Connect IQ app. Post-ride analysis is more precise on the server along with some data filtering logic.
However, I think you may need to tweak your signature, or perhaps it was not synced to EBC before the ride. 
Post the BT report or the Previous chart shown under the activity details.
Was it a non-stop series of road sprint intervals?
Yes it was, this is basically what my intervals always are. Not complaining, it has completely transformed my racing. Just earned enough points to get my cat 1 upgrade after being a 2 since 2014.
2 Likes
When you achieve a BT, your updated signature is applied to the activity’s power data which downgrades the XSS values you saw during the ride.
With the higher TP the watts add up to lower values since you’re stronger than what EBC thought you were during the ride. 
That was some serious hammering especially during the first block that reached Challenge Level 10 and triggered the BT. Looks like multiple BTs in your case which you can see by viewing the Previous chart (pre-BT) under the activity details.