What you are looking at is a cross-reference. In other words the OEM oil cross references to x brands 2.5wt. This does not mean that the OEM oil is 2.5wt. It simply means that brand Xs corresponding oil is 2.5wt. Fork oil is a funny thing in that different brands wt rating does not necessary correspond to another brands wt rating. It's not as stringently regulated as engine oil and as such it varies.
Yes i agree with you, i discovered on the charts that the SAE Oil Weight was different from one brand to another, and sometimes the difference is important.
In fact, i was looking at the oils that were available in my local shop, which are motul and motorex, and in both case, their own SAE Oil Weight grade best matching the OEM oil viscosity is 2.5w (motorex racing fork oil 2.5w and motul factory line very light)...
No I did not bother to look at the charts. I know that is 5wt because that is what it is. I work on these bikes everyday.
It's not absolutely true anymore to say it's a 5w, because as you said, it's really dependent on brand, whatever the grade.
It's for example amazing to see that castrol 10w is lighter than some 2.5w...
So i guess that with the oil brand you are using, it matches 5w.
To conclude i will say that i will not rely anymore on the SAE Oil Weight, but only on the real viscosity of the oil !
A nice wiki page on the subject, for people interested :
Suspension fluid wiki