There is a lot of confusion with Group items and that group item’s total amount line.
There are Group items and there are Subtotal items. These work differently by design.
Group items have a line at the bottom of that group. That line is a total for that group item and that group item only. A Subtotal is a total for all items directly above the subtotal, up to the last subtotal. Subtotals ignore any subtotals above them. In other words, subtotals add everything up since the last subtotal above it.
When it comes to using subtotals, the total of a group item is considered one amount. If you have an invoice with just three group items and a then a sub-total, the subtotal will add up all three group items.
Tip: We have no way to subtotal the subtotals. We have had clients ask.