In my last '36 US game, I easily got 10 spies into Canada - while getting 1 spy in that equivalent time in the UK. Took years to build up to 10 spies in the UK. (found Australia harder to infiltrate with a spy too...)
I concentrated on Tech stealing - and didn't care about relations. (didn't matter - UK and Canada for instance would influence me back up to close to +200% every time on their own). Over the next 6-7 years, Canada succeeded IAW the approximate % chance of doing it. (until they ran out of stuff to steal...), but I NEVER was able to steal a tech from the UK. I tried at least 70+ times (and I knew they had stuff to steal...). Found Japan easy to steal from, Canada easy (would succeed 1 in 4, 1 in 5 times), Germany moderate (1 in 10), Australia never succeeded (tried +50 times without success), UK NEVER.
So my conclusion was that the %ages are base numbers and these are modifed by the ministers profile, tech, or were caused by some bug.