The metallicities of open clusters (OCs) are probes for many Galactic disc properties, such as age-metallicity relation and abundance gradient evolution. In recent years, a large number of newly found OCs or OC candidates have been reported. However, the metallicity values of OCs in the Galactic disc in different works vary from each other. In particular, the cluster metallicities in our previous work (hereafter LISC I), which are determined from Gaia's photometric data, are obviously smaller than most other catalogues. This work aims to investigate the metallicities of Galactic disc OCs in LISC I. We redetermine the mean metallicities of 79 OCs using the spectroscopic data of Large Sky Area Multi-Object Fiber Spectroscopic Telescope DR8 and get obvious discrepancies between the two works. The causes of the difference are studied by using two different isochrones and uncertainties in colour-magnitude diagram fitting process. We find that the evolutionary models adopted and, to a lesser extent, the fitting methods are mainly responsible for the discrepant values of LISC I metallicities. We also propose a linear relationship to correct for the LISC I metallicity, which should only be used to provide more reliable [Fe/H] values in the absence of spectroscopic determinations.