Alkalinity and pH are related, but not the same. It is possible to have high pH and low Alkalinity. To put it simply: pH is a measure of the ratios of H3O+ to OH-, and Alkalinity is a relative measure of a number of different synergistic factors.
Check out the chemistry forum and the Chemistry posts by Randy Holmes Farley in the last sticky topic, and there is a thread in there relating Alkalinity to pH.