The contrast ratings are based on scenarios that would essentially never be used in any theater. They are likely valid and real measurements, not just representative of real-world performance.
They are made at the longest possible throw distance (which maximizes contrast), and with the Iris completely closed down. The net light output would be so low as to be impractical with any but the tiniest screens.
So, with a shorter throw distance, and the Iris open, or nearly so, there will be contrast differences, but much smaller, and much harder to detect: say 30,000:1 vs 25,000:1. I use those as examples, as I don't recall the precise measurements that have been made, but those are likely pretty close.
I owned the NX7, and had a pre-production NZ7 for a 'first look' in my personal theater. I have a 160" diagonal 2.35:1 screen, with 1.3 gain. The bulb on my NX7 had perhaps 1100 hours on it, so wasn't at full capacity. But the light output from the NZ7 made a big impact on my picture quality, especially with HDR content. I made the decision then and there to move to the NZ series, and eventually ended up with the NZ9.
So depending on your light needs, that added output of the NZ7, along with its long-term stability, not dropping as the bulb ages in the NX7, would be much more important than the contrast differences used with your actual throw and Iris setting.
Hope this helps!