Please forgive me if this post is in the wrong place or if this has already been noted; I love the deckbuilder on this site, but I've never posted before.
It appears that there's a slight issue with the average mana cost display for decks. Several times I've had the display show one value on the "Overview" tab (to the right, beneath the total $ cost), then a different value on the "Details" tab (top left, above the graph, where it says "Mana Curve"), and then a different value at the bottom of the "Details" tab (the avg shown at the very bottom between total cards and total cost). After doing the math manually, I've found that the value on the bottom of the "Details" tab tends to be correct.
In case it helps, I linked a deck I was working on recently, that produces the issue. The actual average is exactly 2.6 (no rounding). However it shows up as 2.58 on the "Overview" tab, as 2.61 above the graph on the "Details" tab, and (accurately) as 2.6 at the very bottom of the "Details" tab. I thought one possible factor could be that several of my decks have mana cost overrides on a few cards, but if one of the displayed averages wasn't taking the overrides into effect, the numbers would be even more skewed then they are.
Thanks for your time and also for this site.
https://deckstats.net/decks/64561/855577-swolbriar