SMA 75 and 50 ohm difference
The 75 ohm version of a 50 ohm connector (at least for the SMA, N and BNC types that I've studied) raises the impedance in two ways, while keeping the mechanical interface identical.

a) Reduces the diameter of the pin through the connector, while keeping the diameter of the mating part. This increases the ratio outer/inner over most of the pin.

b) Reduces the length of the dielectric support in the connector. 

Both of these methods increase the impedance of only part of the path through the connector. At low enough frequency, you could regard this as a simple averaging of impedance. 

For high frequency work, you need to analyse the effect of sections of transmission line with different impedance. The high impedance line looks inductive, the low impedance line looks capacitive. The combination looks like a low pass LC filter. With the correct choice of lengths and impedances, the filter is matched. As the deviation from ideal impedance, and the length of the deviant section reduces, so the corner frequency of the filter increases.

It's the relatively large intentional impedance variations in a 75 ohm version of a 50 ohm connector that give them a smaller bandwidth than their native 50 ohm version. If you want high bandwidth 75 ohm connections, you need to use a connector that's been designed for that impedance from the ground up.