If your Leaf Spy numbers don't agree with a gauge, don't blame software.
It's the sensors in the wheels that vary.
Just as tire gauges can vary from model to model, from unit to unit, and from new to old, internal tire pressure sensors can also vary. The four that are on my car are very good. Others have said the same thing. Yet others have seen variation.
These sensors vary statistically with most being excellent and a few being off like tails of a standard normal distribution or like outliers. Even though they are calibrated at the factory, shocks from driving, material aging, and other factors increase tolerance beyond initial calibration limits.
YPMV (your pressure may vary)