It must be me ... mustn't it ... ?

The lead-in to BT speedtest seems to have changed.

There is now a whole series of nine steps to be gone through before testing. OK, some of it is entirely logical but step 5 says ...
Reboot your modem/router by powering it off,waiting one minute,and then powering it up again
...etc etc. Then step 8 instructs ...
Ensure the 10 day stabilisation period has completed before reporting any suspected fault-this is how long it can take for your Broadband to train to its optimal speed.

So, am I wrong is it there something illogical here? Like if I must reboot every time I try to test?

They are making two assumptions.

First, that you are doing the test because you feel you have a slower speed than you are used to.
Second, that you are on ADSL2+.

The reboot is to ensure that you haven't got a low sync for some reason, a random burst of noise such as a thunderstorm for example.

The second logically follows because nowadays a reboot/re-sync on WBC ADSL2+ causes an immediate rise in IP Profile. So they are trying to ensure any temporary problem is eliminated, so as to narrow things down to more serious problems.

This falls down for a number of ISPs. Including at least AAISP, all resold Entanet products, and Plusnet, where they hold a copy of the BT Wholesale IP Profile which does not react immediately. Possibly taking several hours or even failing to update.

So if you know your sync is OK, ignore it.

Step 8, as ever, is tripe. But is rigidly adhered to by all ISPs as no-one in BT Wholesale seems to understand their own system. As a result ISPs' staff have no avenue open to challenge it. To the point that BTW and BT Retail training clearly teaches newcomers the same garbage. The teachers don't know it is rubbish.