lets say ur lens is 18-70mm f4-5.6. That means at 18mm, the biggest appeture u can use if f4. At 70mm, the biggest appeture u can use is f5.6.
BUT from 18-70mm, u can use even smaller apetures. for example at 18mm, u can also use f11 or f22, at 70mm, u can also use f11 or f22..
There is a limit on the biggest apeture u can use, but not the smallest one..
for those lens that say 70-200mm f2.8, this means that throughout the range from 70-200mm, u can use the big apeture of f2.8