Average grade vs Overall Covered Coverage

While doing code coverage analysis, in the coverage tool we see two numbers,
one saying Overall Average Grade and Overall covered.

what is the difference between these two numbers?

Let’s see with one Code Coverage analysis example.

Consider Block and Expression coverage analysis for a design,

Assuming it consists of 73 blocks and
85 expressions.

Considering 69/73 blocks covered.
74/85 expressions covered.

so,
Block Coverage          is 69/73 = 94.52%
Expression Coverage is 74/85 = 87.06%

Overall Average Grade = (Block Coverage + Expression Coverage) / 2;
= (94.52 + 87.06) / 2 = 90.79%

Overall covered = (No. of blocks covered + No. of expressions covered) / (No. of blocks + No. of expressions)
= (69+74) / (73+85) = 143/158 = 90.51%

Overall Average Grade = 90.79%
Overall covered            = 90.51%