Pressure at the pump is not that relevant. What is relevant is the pressure at the point where pressurized water meets the coffee puck. And this pressure very much depends on multiple factors but let's say that the coffee ground level is one of the most important things. If you have a pump pumping water at 15 bars but you have a very coarse ground coffee in your basket, resulting pressure is _not_ going to be 15 bars. Similarly, if you grind the coffee at very fine levels then the resistance of the coffee puck will increase and therefore resulting pressure might very well end up being _more_ than 15 bars.

That said, another thing which plays a big role in the resulting pressure is the design of the brewing head and the design of your basket. Baskets can be either pressurized (they have a single hole through which the coffee will flow) or non-pressurized (they have a lot of small holes through all of which coffee will flow). You can imagine that the resulting pressure will depend on the type of the basket you're using.

Because cheaper appliances will by default use pressurized baskets, they will tweak the pump to some pressure level which they found to work the "best" for their type of machine and that level is exclusively much more than expected 9-10 bars. Nespresso will for the same reasons do the same with the difference that they make one step forward to take over the full control over the puck preparation process through their prepackaged pods.

Nespresso's taste is least likely to be anything anyone is looking for in a good espresso.

That high pressure causes some odd taste notes and it would cause channeling in the puck. Not to mention all sort of other issues which start already at ~11 bars.