This antenna:
That’s because internally they usually use a sleeved dipole for single band use or circuit board etched dipoles for dual band use. A dipole has a gain of 2.1dBi, the “i” is in reference to a theoretical isotropic radiator that radiates equally in three dimensions. All omni-directional antenna gains should be in reference to an isotropic radiator, and all directional antenna gains should be in reference to a dipole, as a dipole is often the primary radiator in a directional array. However many manufacturers will use artistic license and omit the “d” in dBd and use a figure based on an isotropic radiator because the numbers are larger by 2.1dB.