In order to justify their existence and salaries the IET boffins must continually find new superfluous regulations and changes to impose on the industry. Otherwise they are out of a job.
No other reason than that.
I think it was the "clocks change" argument for the 6-months label as you see different RCD saying monthly, quarterly, regularly, etc. I think the reality is they are tested when EICR are done and that is about it!
It is one of the reasons that I don't approve of ignoring the Zs or R1+R2 values and assuming the RCD side will cover their asses on any fault, the RCD should be for additional protection, not as the only means of disconnecting in under 0.4s because of poor design or degradation of the installation, since RCD electronics is more complex and they don't get tested in practice.