VFP doesn't handle numbers greater than 16 digits of precision (left and right of decimal point). It rounds from there.

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Visual FoxPro Documentation - Using Visual FoxPro - Programmer's Guide - Appendix - Visual FoxPro System Capacities - Digits of Precision in Numeric Computations

>I've been trying my index expression after changing field type from numeric to currency: STR(price,16,2).
>As -922337203685477.5808 is the low limit for CURRENCY type numbers, I tried it in a command window:
>
>x = -$922337203685477.5807
>wait window STR(x, 21, 4)
>
>I expected '-922337203685477.5807' string but got '922337203685477.6000'.
>After some investigation I found that STR() prints only 16 most meaning digits rounding the rest:
>
>
>str(-922337203685477.5807,21,4)=='-922337203685477.6000'
>
>str(-022337203685477.5807,21,4)== '-22337203685477.5800'
>
>str(-002337203685477.5807,21,4)== '-2337203685477.5810'
>
>str(-000337203685477.5807,21,4)== '-337203685477.5807'
>
>
>Or maybe it's an effect of some misterious SET?