Compressibility causes the ASI to over read. EAS is CAS corrected for compressibility so CAS will always be higher than EAS. At low speeds (i.e. below TAS 300kts) the difference between EAS and CAS is so small that we can ignore it but it is still there. If you climb at constant EAS then CAS will increase (as TAS and, therefore, compressibility increases). If you climb at constant CAS then EAS must decrease for the same reason.