The thickness of a tile gives some guide to dating tiles but only some. Porcelain tiles may be even thinner. Several companies made thicker tiles for hearths and many attempted to sell thinner tiles to reduce costs.
Tiles can vary quite widely in height and width dimensions but even more so in thickness.
The amount of clay that went in to the press, it's moisture content are firing shrinkage would all contribute to size diferences. Thinner tiles are often handmade from plastic clay and if dust pressed are often a red or brown clay which are stronger than white clays. There are thinner white clay tiles but not many, Rhodes were apparently successful as many of their tiles are close to a quarter of an inch thick but they were only in business for about seven years. Craven Dunnill had a patent back which was hollowed out, it looked three-eights thick on the edge but wasn't in the middle. Maws, Sherwins, Pilkingtons and Copeland also made thinner biscuit, thicker was done by Mintons, Minton Hollins, Copeland, Richards and most art potteries.