USS Error and where to get advice?

RSTime

I have finally received a reply from USS. 

"After reviewing your documents, I can confirm that the settlement letter you received on 06 August 2024 contain the correct figures following confirmation on your option forms that you have previous Lifetime Allowance usage of 18.17%. Taking this into consideration at the time of settling your benefits, a reduced maximum tax-free sum was allowed compared the quote which assumed no previous usage."

I have started a smaller TPS pension equivalent to a LTA of 18.17% (with no lump sum). Is the USS response correct given I have not taken a lump sum from my TPS pension, would I expect a reduced maximum tax-free sum on my USS if I have already started my TPS pension? I am nowhere near the maximum lump sum of £268,275 with my USS (when there are tax implications)? 

Can anyone explain the USS rationale for reducing my maximum tax-free sum?

EDIT - Given the USS response I have started a new thread.