might be the, femur on your leg is long, but the setback, seat post offset ,
available from your current seat post, is too short, and the angle for the seat tube is too steep ..

Brand/model of frame is a mass production , the body of the rider is a variable..

the 5mm may be imperceptible, .. on its own..

i had a zero set back seat post , and a B 17 saddle, the combination of a saddle
with the ability, of a longer rail adjustment range, to shove it back, further ,
and a crank arm change from 170 to 180 gained an inch.

kept the seatpost, as it had a quality of suspension function