Normalized values provide the greatest precision for a given number of bits. However, there are a large number of non-normalized values which we can represent with the 80 bit format. These values are very close to zero and represent the set of values whose mantissa H.O. bit is not zero. The 80x87 FPUs support a special form of 80 bit known as denormalized values. Denormalized values allow the 80x87 to encode very small values it cannot encode using normalized values, but at a price.
Denormalized values offer less bits of precision than normalized values.
Therefore, using denormalized values in a computation may introduce some slight inaccuracy into a computation. Note that the 80x87 status register contains a bit you can use to detect when the FPU uses a denormalized value in a computation.