The difference between the ABSOLUTE top spec and the absolute lowest spec is 20.8%. That's from the very top to the very bottom. So "over 20% in some cases" is accurate only if you are counting from the #1 spec to the #32 spec-- any other gap is less. (and that lowest spec has a spec of the same class that is much closer).
Based on what evidence?
Looking at some specific fight samples you regularly see deltas greater than 20%
25H Jin'rokh, all: top-218k, bottom listed-148k, difference ~30% (bottom overall is 135k with insufficient parses)
10N jin'rokh, all: top -162k, bottom-111k difference >40%
25H Primordius, all: top 227k, bottom-141k difference > 40%
10 N Primordius, all: top-146k, bottom-105k difference ~40%
That clearly looks like > 20% "in some cases" to me.
Now if you mean "average overall dps" then yes, the range from top to bottom is right sround 20%. That just means either the fights that favor certain specs REALLY favor them or there is a pretty even distribution on how fights favor specs. However, it doesn't alter the fact that on average there are differences between specs up to or greater than 20%