I think the early sender units (and gauges) had different resistance. It could also be something mechanical with the float, there should be enough of a stub sticking out from the float arm to act as an indication or to get hold of with mole grips for some "manual stimulation".
Credit to DKLawson, who used to post on TMF:
“Minis before late 1964 had the 0-90 Ohm (empty to full) range that you mentioned. That is comprable to some GM guage systems.
From 1965 on the classic Mini had a sending unit with a resistance range of about 270 Ohms = Empty to about 30 Ohms = full. These are nominal values. If you measure a Smiths sending unit it probably won’t match these values exactly.
To use an aftermarket gauge with the bayonet mount sending unit you will need a sender meeting the industry standard range of 240 Ohms = empty to 33 Ohms = full. Your gauge will show empty a bit sooner than true empty but consider that a safety bonus to prevent you from running out of fuel.