Reward seat discrepancies: internet vs phone

Many of us have seen a G online, only to discover it's not actually there when we phone up to snag it even if we're looking at the G whilst on the phone.
I was given quite a detailed explanation for one reason this might happen by a VS FC customer service agent last night. I was looking for some NYC-LON M-G seats, so I'll use that as my example.
She said it's because for r/t bookings there are separate/discrete (and perpetually shifting) allocations for US-originated reward seats, and for UK-originated reward seats.
Frequently I look at one-way dummy bookings to see what reward seats are available. I saw one G each on my two preferred flights (JFK-LHR; LHR-EWR) and phoned up, only to find the inbound leg of the US-originating r/t (LHR-EWR) wasn't available... except that it would be available if that leg was an outbound sector of the r/t (i.e. a UK-originating r/t). There was nothing she could do about it!
I'm sure some people here know this already, but it was a surprise to me!
I was given quite a detailed explanation for one reason this might happen by a VS FC customer service agent last night. I was looking for some NYC-LON M-G seats, so I'll use that as my example.
She said it's because for r/t bookings there are separate/discrete (and perpetually shifting) allocations for US-originated reward seats, and for UK-originated reward seats.
Frequently I look at one-way dummy bookings to see what reward seats are available. I saw one G each on my two preferred flights (JFK-LHR; LHR-EWR) and phoned up, only to find the inbound leg of the US-originating r/t (LHR-EWR) wasn't available... except that it would be available if that leg was an outbound sector of the r/t (i.e. a UK-originating r/t). There was nothing she could do about it!
I'm sure some people here know this already, but it was a surprise to me!