I had that happen once on an old Volvo engine in my 1968 wagon. It turned out to be a clogged pcv valve. If the crankcase can't vent it will find the next available opening; it has to vent somewhere eventually. If crankcase ventilation is working then my guess would be ring blow-by. A compression tester could read low for more than one reason. A leak-down tester is made to pinpoint the problem more precisely.
