IRQ is disalbed, but ISR is called, Why?

HI,

UART is interrupt driven. When IRQ for the UART is asserted, its ISR is called. In the ISR, after it reads buffer, it raises an Event. A separate Thread waiting at the Event activated when the Event is posted. In the Thread, it disables IRQ for the Uart. Then it does further processing. I expected that next IRQ will not be served until its IRQ is re-enabled again. But it seems to me that, the ISR has been called during the IRQ is disabled. Can anyone help me explain why?

Thank you,

David