Why this evaluation board use different track impedance than the right one? Ethernet PHY, routing
Using exactly the same values as you did, but with another calculator (from EEWeb), I get:



Which is different from what you had, and closer to the target. There are several models that can be used to estimate the differential impedance, and different tools may use different underlying computation techniques, so you can get different results.

In any case, 83 is within 20% of the ideal 100 ohms value, so you're in the right ballpark. If you need tighter tolerances anyway, you would need to ask the PCB fab house for impedance control, which implies that the fab house itself adjusts your layout (trace width / separation), according to the characteristics of their materials and fabrication process. So you don't have to worry about relying on the results of a random PCB calculator tool: instead, you specify the target impedance to the PCB fab house and they take care of it, eventually performing time-domain reflectometry tests on the actual produced boards to check it.