What's the difference between Carvacrol and Thymol?
On a reverse phase C18 column  (which itself is hydrophobic, it is an octadecyl hydrocarbon chain bound to the support material) the more hydrophobic the molecule, the more strongly it will bind to the stationary phase [like dissolves (or in this case binds to) like], and the longer it will take to elute with an organic solvent. Below are the structures of carvacrol and thymol.  The more exposed the hydroxyl group, the less hydrophobic the molecule.  The hydroxyl group in carvacrol (ortho to a methyl group) is more exposed than the hydroxyl group in thymol (ortho to an isopropyl group) because of the relative steric sizes of a methyl group vs. an isopropyl group. Therefor thymol is more hydrophobic than carvacrol.  Consequently we would expect thymol to bind to the column more strongly than carvacrol, and therfor we would expect the carvacrol to elute first which matches your observation.