I believe some of the terps in cannabis are more water soluble than others. Not all but some. Also, there are probably other things in rosin. Perhaps chlorophyll or other stuff from the pot.
"Each terpene will have its own level of solubility in water at a given pressure and temperature, with the more oxygenated ones having better solubility than the non-oxygenated ones or less oxygenated ones. For instance, if you added a solution of equal parts Myrcene, Linalool, and 6,7-Dihydro-7-hydroxylinalool to water, mixed, and allowed time for proper decantation, you would find Myrcene at the lowest concentration in the water, Linalool in the middle, and 6,7-Dihydro-7-hydroxylinalool at the greatest concentration."