While there are other ways also to do it. One very simple way is to use Quantity as a dimension rather than Expression because in your scenario it is working as a dimension only.
Or if you want to put it into expressions then you can simply use avg function also rather then using sum, sum here does not make sense because you don't want to sum the quantities.
So, just use avg(Quantity) then you will see correct data only.
Thanks Ashutosh, it worked like a breeze, it just takes a while to understand how QV set analysis functions work and you are correct I was using the sum incorrectly in this context!
I had multiple sets of such data with various conditional computations on the Quantity required and displayed in
the charts, hence I had used expressions, here is the formula I managed to use to get the average of quantity aggregated by bookingid and did the sum - sum(aggr(avg(Quantity),BookingID))
All my Quantity based computations are working fine now.
Thanks again for your help!!