At least that's what is being reported.
http://www.profootballtalk.com/2008/03/27/seahawks-to-add-mare/
Mare was good on kickoffs and poor on FG. But like John said "field goal percentage is a poor measure of a kicker".
Update: Look at it like this: In his first 5 seasons Brown kicked 145 field goal attempts and converted 116. He's been 80% accurate. In Mare's first 5 season he kicked 161 field goal attempts and converted 136. He was 84.5% accurate. After that Mare had 3 straight seasons where he didn't top 80%. Now, did Mare get worse or did the larger sample size simply give us a better idea of Mare's true ability? Meanwhile, Brown has kicked 417 kickoffs. He averages 62.5 yards and kicks a touchback once every 11.1 attempts. Mare has 774 kickoffs. His career average is 63.9 yards with a touchback every 4.5 attempts. In the last 6 seasons, 392 attempts, Mare has been even better. He's averaged 65.1 yards per attempt and a touchback once every 3.8 attempts. That's something you can hold your hat on.