Tyre Sizes???

Hi To All,
i have been looking at fitting a larger rear tyre. If i look in the SIP catalogue i see the equivalent tyre to a 3.50 X 10 is a 100`90-10.
Now this is the confusing bit!, going by the example that is in the catalogue, these two sizes DO NOT equal each other.
I.E. 3.50 in inches is NOT 100mm, 100mm is actually 4 inches.
So my question is am i actually working this out incorrectly or is there another explaination?

I actually want a tyre about 3.75 inches wide to fit a 10inch wheel, so what size is that in metric?

thanks for any help
DTC
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