I would guess they are old systems. I bought 1st house in 1972 which was piped like that, but it was going out of favour vs boiler - open vent - cold feed - pump then or shortly after, due to the issue of seesawing causing aerated water entry. I modified it, and piping was simpler as the cold feed point was then directly below the F/E tank.Virtually all open vent systems (except gravity to cylinder) I know of in my area are done with the vent off the flow at cylinder and feed off the cyl return.