I think one of the reasons to use the same process numbers is because there are only a few suppliers for fabrication equipment (like Applied Materials, teradyne, etc.). Wit the cost of IC processing shooting up rapidly, the only way for IC fabrication being remotely affordable is for companies to place orders for similar pieces of equipment to lend economies of scale. That's also the reason the industry seems to move pretty much in lockstep (+/- a few months).
As for using greater than min length devices, there's quite a few reasons. Firstly, to make a device weaker than the min size device, you can't reduce the width any more (Limited by process). You have to increase the length. This is important in things like startup circuits (where you want some devices to be very weak). In addition, matching is strongly dependent on the area of the transistors. Min length transistors also have a ton of Vt variation with corners and bias points because of short channel effects like DIBL (Drain induced barrier lowering), and punchthrough. Finally (and most importantly for an analog designer like me), longer devices have higher output impedance for the same current levels, which allows you to build amplifiers with higher gain.