Short answer: The value returned only has three statistically significant digits. Due to the way a computer handles non-integer numbers, though, it will display the number with as many digits of precision as it can generate unless you explicitly tell it otherwise. To do this, look at the VB FormatNumber function.
More info: Numbers are stored in computers in one of a few well-defined formats. In the case of non-integer (floating point) numbers, this is usually either a 32 or 64 bit representation. In the 64 bit representation, for instance, I believe 53 bits are used for the mantissa and its sign and 11 for the exponent and its sign. This gives it a precision of log((2**53)/2)=~15 digits. There is no space in a computers standard representation of
numbers to dictate which are statistically significant digits so when it is asked to display a number, you get the whole 15 (minus any trailing zeroes).
OK, so you may ask "why aren't all of the trailing digits except the significant 3 set to zero?"
The ADC accuracy a set number of bits. A 12-bit ADC will give you 3 digits of accuracy for unsigned numbers (log(2**12)=~3). The ADC communicates its reading to the computer as an integer from 0 to 1024. The computer takes this integer and scales it to the actual range of the ADC (lets say 0 to 10.0 V). To do this, it uses a formula like y = (10.0/1024)x. Where y is the scaled voltage and x is the integer reading from the ADC. Because the computer doesn't know anything about statistical significance, 10.0/1024 = 0.009765625 and thus the number that would be returned in this case would be an exact multiple of that.
I hope that makes some sense. In general though, go by the stated precision of your card and use the FormatNumbe
r function to round it to the correct number of digits for display.
TonyH
