Hey, I have the same problem, looks to be something rare:
Pool built in 2003, on my second board after lightning strike long ago.
Pool reads 1800. Actual level is 3200ppm.
Cleaned Cell. Still 1800. Ordered new cell. Still 1800. Had replacement for new cell. 1800.
Fixed original board (thermistor) and re-installed. You guessed it, 1800ppm.
Been reading these forums like crazy. Changed the fuse. Up to 2200 now. Still far short. (But a 20% increase!)
My question: Could my fuseholder be offering resistance? My amperage reduces slowly from 4.5 to 3.84.
(This happens as the instant salt reading trickles down. Is the salt level calculated by the relationship of the volts to the amperage?)