Because for a long time now every single success has come back super fast.
I know you cannot say either way on that alone but for me it's enough to make me suspicious.
Colin does alot of work outside the UK I just don't see how they can all come back singed genuinely so quick.
I wrote to him a few years ago and robustly defended them as real then but since then I have questioned this.
It's worth saying I am not alone in raising suspicions that Colin's successes are secretarial.
Someone mentioned Judi Dench email successes are believed to be authentic and they come back fast. That was a good point and indeed something to consider. But she mainly works in the UK now, more UK work than non UK IMO.
Another example is Josh Dallas. A high volume of requests come back fast even though he is spending alot of time in Canada filming. The requests go to a US town where he grew up- so it's possible he is visiting there and signing mail, but from the volumes and speed of return I have doubt about this now. Some time ago I pointed out the above as grounds for them being genuine. But since seeing the succession of quick successes I have doubt.
I could well be wrong but I have my suspicions is all

This topic has given me more to consider and opened my mind a little.