What do you mean by just out of reach? How high is too high?
As for them being too low, it's up to your manager to adjust the goal up or down to reflect changes on your route. Examples include a new housing or business development that generates lots of stop volume in a fairly small geographic area, the addition (or loss) of high volume stops that require(d) a lot of time to process, a major long-term construction project that eats up a significant amount of time, the addition/loss of an outlier stop (or stops) that are (were) frequent enough to have an impact on your monthly SPH total, gaining/losing overflow from/to another route, and so on.
There's no perfect way to calculate what SPH should be. The most common reasons (in my experience, at least) for it being significantly out of whack one way or the other are that it hasn't been adjusted to reflect changes in the route and courier complacency or habit.
I'm sure they exist. I've been on many checkrides and seen safe, courteous couriers and swings make fools out of established SPH goals. Nine times out of ten they were simply better organized and worked quicker.
Then there are those swings who never, ever "get" it...