Is it really a surprise
that the 'ablest' negotiators are the ones who have the most leverage?
For smaller organisations or those in a more competitive landscape flexibility is a requirement to win business and so sticking to pre-defined positions come what may is unrealistic (unless you want to go out of business when noone buys from you).
I'm not sure any lessons can really be learnt from the fact that Microsoft are the least flexiblity negotiators because that stance will lose them very little business and so makes perfect sense in their case.