This paper investigates the effect of differential aperture loss with Sloan Digital Sky Survey (SDSS) fibres and examines whether such selection bias would result in the observed correlation between rest-frame absorption equivalent width of Mg ii absorbers, Wr(2796), and mean associated [O ii] luminosity, , in SDSS QSO spectra. We demonstrate based on a Monte Carlo simulation that the observed Wr(2796) versus correlation of Mg ii absorbers can be well reproduced, if all galaxies found in deep surveys possess extended Mg ii haloes and if the extent of Mg ii haloes scales proportionally with galaxy mass as shown in previous studies. The observed correlation can be explained by a combination of (1) the known Wr(2796) versus ρ anticorrelation in galaxy and Mg ii absorber pairs, and (2) an increasing aperture loss in the 3 arcsec diameter SDSS fibre for galaxies at larger ρ. Galaxies at larger projected distances produce on average weaker Mg ii absorbers and weaker (or zero) in SDSS QSO spectra. We show that such a correlation diminishes when larger fibres are adopted and is therefore not physical. While under a simple halo model the majority of Mg ii absorbers do not directly probe star-forming discs, they trace photoionized halo gas associated with galaxies. We show that because of the scaling relation between extended gas cross-section and galaxy mass, the number density evolution of the Mg ii absorber population as a whole provides a good measure of the cosmic star formation history.