I would look at the RT to see if there is some sort of evaluate formula that might impact the count.
I would address this in 2 design in 1 of 2 ways.
Either do the conditional suppress on the details and recreate your own RT to make it work as expected.
Or
Create a formula field to group the data pre-date and post-date...
If table.date<Date(2007,01,01) then 'Pre date row' else 'Post date row', then group on this formula field atG2 (under the customer group of G1) and conditionally suppress the G2 header and details for it being in the 'pre-date' group.
This will also need to use a RT to SUM the value. You can have it set to Reset on Group1. You can place it once on the GF2 and details row. It will then show the predate SUM on teh GF2 and 'roll' into the details for post date rows (not suppressed) finallint onto the GF2 of the post date group2 for the final SUM of that Customer.
Hoep this makes some sense.