Why are there two average order values?

Because they answer different questions. Gross AOV is what customers were charged, VAT included and before refunds, which is the right figure for free-shipping thresholds and basket decisions. Net AOV is what survived tax and refunds, which is the figure every margin uses.

The gap between them is roughly your tax rate plus your return rate. Quoting one while computing costs against the other is a common way to produce cost percentages that look impossibly good.