KLE tank suddenly larger ???

Yes, for the last 8 months (since purchase) the tank could only take 13 litres - no matter how much i shook and tilted the bike - giving me 260km.

On the last fill it took 12 litres after about 250km then started to jurk, filled up as per ussual - it took about 12L again.
Then the fun started - reached 260 - no reserve yet - started to get seriously worried as I was once stranded without petrol after doing 270km. Eventually reached 295 without reserve and went to fill up - and suddenly (for the first time) the bike took 15L.

The fact that I reached reserve after the ussual 260 and then filled up with about 12L and then reached 300km - tells me the extra petrol was hiding at the 'bottom'  and not on 'top'.

Any ideas of what went wrong or right ????? (The book says the tank is 15L) ??? ??? ??? ???