With a properly adjusted limber chain and correctly inflated tire, 60 HP at the rear wheel translates to 68 - 75 HP at the crank. On a cold dry day with properly tuned intake and exhaust, you can get that from a stock displacement engine.
Now, as for the 136 reading. Some dyno's have too many adjustments for the operator to fiddle with. On the newer ones, you can adjust for altitude, humidity, etc to get a reading that can be close to what you would get at sea level with standard temperature and pressure. On some, you can also dial in a drive train loss 'fudge factor' that might give you a rough estimate of crankshaft power IF it is set correctly.
So, I'm thinking the dyno wasn't setup correctly, but it seemed to be more accurate on other bikes because they had under inflated tires, incorrectly adjusted belt drives and cold, heavy transmission oil, all factors that will result in low readings.
