I believe the residency requirement was set as it was more difficult to do background security checks on people who had lived overseas. I would speculate that discretion might be more likely where there are existing security agreements, say Australia. I can't imagine a year in Russia would be seen in the same light.
I don't know why the three services have different requirements. This could be historic as I think the RAF had more officers in sensitive posts than Army or Navy
and it is easier and safer (for your career) to carry on as before.