For any NMR active element, the chemical shift is given relatively to a reference compound. For 29Si the most common reference is Me4Si (TMS). This means that TMS will have a chemical shift of 0.  Then the chemical shift of other compounds relatively to TMS will depend on the relative shielding (higher shielding will result in negative ppm)

The chemical shift in NMR depends upon the electron density on to the element of study. Although a more electronegative element (oxygen) is attached with silicon. The extended coordination in silica-alumina or zeolite probably increases the electron density on silicon, which results in shielding of 29Si peak.