Variance in liquid malt extract colour in the paler kits is common due to batch to batch variance along with natural colour pick up over time.
This is due to Maillard reactions, a type of non enzymatic browning which has a similar effect to caramelisation.
The colour pickup occurs over time and is accelerated at higher temperatures, so if the kit has been stored in a warm shop or warehouse it would be darker than if stored in cool places as we recommend.
The beer will still be fine to drink, though the darker/older one will have a richer, maltier flavour than the one lighter coloured brew.