There are very few engines that have a short spec bypass valve rating. Most have a range from onset to full open.
First I thought it was a manufacturing tolerance, but later found out it was a performance spec. Onset to full open.
EXACTLY! People want to think of this as a static singular set point, but it's not for a few simple reasons.
1) the bypass is typically made with either a wound metal spring, or even a flat spring. All these types of springs are most often of simple, linear rate design. (IOW - not a "progressive rate" spring"). Therefore, the more it opens, the greater pressure it takes to displace each linear increment of opening.
2) to have a true singular opening pressure rating, there would need to be an elaborate control mechanism in place that went from a "closed" position, to a "full open" position capable of maximum flow; a sort of "pop full open" device. That would be a very elaborate and complex thing. It certainly could be designed, but who needs such an expensive addition when not required?
3) the bypass pressure is not really all that critical, as long as the range is near factory specs. A few psi up or down does not make a discernable difference to the safe operation of the equipment.
4) there are two important criteria for the filter bypass feature to meet. One - As long as the full flow of the bypass feature can pass enough volume to safely protect both the filter and the engine pump from damage, then all will be well. Two - the bypass will need to open at a reasonable pressure as to assure something does not over-pressure the desired protected components.
So, all the major aftermarket manufacturers have looked at the application, reviewed their product line-up for many criteria including size, flow, efficiency, lifecycle, pressure ratings (both burst and bypass), adbv, etc, and made a recommendation (or sometimes two).
I'll give you a good example of how much "wiggle room" there can be at times:
My Kubota L3430GST used to be only spec'd by Wix for the 51311 filter. Upon the next generation of tractor, they have upgraded the filter choices to either that 51311, or the 51307 filter (coincidently which is the ONLY one recommended for my neighbors L3240GST Kub). Now, the engines in our two tractors are the same except for the HP rating, dictated by the fuel injection system. But for some reason, Wix recommends either filter for mine, but only one for his. The 51311 is rated at 16 psi; the 51307 is rated at 8-11 psi. So my tractor can endure either, but his is only rated for the lower one? I find that dubious. And not for one second do I think that the 51311 has a single-point opening pressure. Go figure ...
My point is that the filter makers know what they are doing and if Wix and Purolator have available products for your Onan, you'll be fine chosing either. I'd trust either brand to make a solid recommendation for any application. Don't sweat the small stuff; someone has already done that for you.