The term 3.5 inch floppy disc is in fact a misnomer. Whilst the specification for 5.25 inch floppy discs employs Imperial units, the later specification for the smaller floppy discs employs metric units.
The standards for these discs are ISO/IEC 8860-1:1987 (double density), ISO/IEC 9529-1:1989 (high density) and ISO 10994-1:1992 (extra-high density); all of which specify the measurements in metric. These standards explicitly give the dimensions as 90.0mm by 94.0mm.
In most countries, the national standard is simply derived from the ISO standard. In the U.S., however, the applicable standard is instead ANSI X3.171-1989. That, too, specifies the measurements in metric, though. It specifies the catridge dimensions as 90.0mm by 94.0mm by 3.3mm and the diameter of the magnetic disc material itself as 85.80mm.
If you have a floppy disc catridge that does not measure 90mm, then you have one manufactured by someone that is not careful about manufacturing tolerances. (And you should beware. If their product isn't up to specification in one area, it may not be so in others.) If the disc measures 3.5 inches, then it is the wrong size. 90.0mm is 3.5433 inches.