Thank you all for your replies!
"The value is divided by 100. 1.00 = 100%" - I thought about it but
1) I haven't found any documentation saying it is divided
2) the value Performance Monitor is displaying is NOT divided - it displays 100, NOT 1
...so it looks like it is the SQL Server itself which modifies the Performance Monitor value from 100 to 1, but it must be completely nonsense...
By the way, as you can see on the screenshots above the Performance Monitor alert is also configured to be <90 but it has NEVER fired!!!
"I never recommend to setup alerts on this value. " - thank you for the suggestion!
"Thus, like Tom said, you should set it to .90 for an alert value." - but why in SQL Server this must be 0.9 whilst in PerfMon 90???
Regards,
Michael