Difference in flow?

Hi, I'm on a gravity-fed system with boiler in garage, cylinder in landing cupboard, and water tank in loft. My water pressure on outside tap reads between 2 and 2.5 bar depending on time of day. The cold mains enters the boiler in 15mm piping. I'm getting about 6 litres per minute out of all downstairs cold taps, including kitchen tap, EXCEPT for my downstairs toilet tap, which is firing out at about 20-25 litres per minute. I've painstakingly checked the pressure on that tap and it reads at 2 bar so I'm hugely puzzled at why I'm getting such a difference in flow. Is it likely that all downstairs taps are on 15mm pipe except for this one and that it may be on 22mm??? Thanks, Nick