The pressure sign topic comes up periodically. Denton Bramwell's
data in this article, in which he used a Pressure Trace to get actual reading to correlate to case head expansion and pressure ring expansion, shows the same lot of cases with the same load history have about a 2:1 range of pressures over which one case or another will show the same amount of expansion. We've all seen primers do this at one time or another, where most look fine and then one pierces.
Below is a table of data from SAAMI that shows the same reference load measured by different copper crushers. Note that the readings vary by 23% from one facility to the next, and this is using extremely uniform calibrated copper slugs and tight tolerance SAAMI pressure and velocity barrels. Now consider that primers and cases are also copper alloys and that by looking for pressure signs in them, you are looking at how they deform. They are not calibrated, nor are they as precisely uniform as the slugs, so how accurate can pressure readings based on their degree of deformation be? This explains how a 2:1 difference in pressure indication of can occur with them.
Basically, a pressure sign tells you what the component or gun being used can tolerate in combination with one another. It is pretty common for a case wall to be a couple of thousandths thinner on one side near the web than on the other, not to mention weight differences from coming off different tooling. So it's not really hard to understand why pressure signs that occur every now and then may not be present at all with other cartridges made the same way.
So, why do you care what the pressure is, short of blowing up the gun? There are two areas of concern:
1.) Will the ammo be used in more than one gun, either intentionally or unintentionally (e.g., a not too brilliant nephew (didn't you warn your sister not to marry his father?) gets their hands on your reloads and tries them in his gun)?
2.) Wear and tear. (How many rounds before the throat is shot out? How soon will lug setback become apparent? Will you have enough additional primer leaks over time to scar your bolt face significantly? (Do you ever expect to be able to sell this gun to anybody else?) Guns designed for a particular cartridge have lifetime considerations for which the designer assumed the normal pressure limit for the cartridge would be observed. There is no simple way to guess how much difference to durability will be made by shooting at pressures over that number.
The bottom line is the one you've seen echoed repeatedly in many places. Find a reasonable load that maximizes accuracy in your gun, as shot placement is more important than power the vast majority of the time. If you need more power, get a gun chambered for a cartridge that delivers it rather than shorten your gun's life in an unpredictable fashion.