Here's why:

* Leading zeros (zeros before the first non-zero digit) are *not* significant.

* Trailing zeros (zeros after the last non-zero digit) are significant *only* if they are after the decimal point.

* Captive zeros (zeros between non-zero digits) are always significant.

In 0.00321, the leading zeros (0.00) are not significant, while the captive zeros (321) are significant.