the problem you are having is that decimal places (ie . something) are in decimal (unsurprisingly) whereas pounds in stones are not. So 10.5 stone could either be 10 stone seven pounds or 10 stone 5 pounds, depending on which converter you use.
On a more general poit, when rounding up, anything 5 and over is rounded up, anything under 5 is rounded down
You are muddling fractions of a stone with fractions of a pound. Since it's not a decimal relationship (1st ≠ 10 lbs) then the fraction changes, and consequently the digits after the decimal point do too.