Block value from strength not calculating right?

According to my tooltip in game, I am getting 513 block value from 1047 strength (1047 / 2 = 523.5).

I then removed all gear and talents, reducing my base strength to 175 (I am an 80 draenei warrior).

The tooltip now showed a block value of 77 (175 / 2 = 87.5).

I equipped a +29 strength weapon which gave me 204 strength.

The tooltip now showed a block value of 92 (204 / 2 = 102).

If I subtract 21 from my strength values across the board I get:

154 / 2 = 77 block value

183 / 2 = 91.5 -> round up -> 92 block value

1026 / 2 = 513 block value.

Is this as designed or possibly a bug related to draenei base stats?