Have you tried asking the question on the Silver Strike BB -- thestrikepoint.com ?
Also, the SS club has a general info page at http://www.silverstrikers.com/
It doesn't answer you question directly, but has some interesting info on strikes.

Just a guess, since I don't collect them --
IF those $28 strikes were used during the same period as the $7 strikes, it may have been that the $7 strikes were won on a 25c machine, and the $28 strikes won on a $1 machine. That would be the same payout ratio: (25c >> $7 strike) x4 = ($1 >> $28 strike).