I've been wondering at bit about that. My hose internal Diameter along the whole run is 1/4", which fit over the metal pipe sections, which are, ext diameter 1/4" / int 5mm.
My plastic filter, apparently for 1/4" hose, has only int diameter of only 4mm.
Likewise, an inline fuel tap, I've recently fitted in the boot (as an anti theft device), for 1/4" hose, is only 4mm, int diameter.
Dave