I don't think you're seeing anything unusual, it's normal for the way Windows (or any protected mode OS) operates. Devices are virtualized in protected mode, so functionally the Device Manager is referencing little in the way of hardware IRQs. The Hardware Abstraction Layer creates virtual IRQs to access the devices on the system, and on a mainboard that implements ACPI (Advanced Configuration and Power Interface), Windows will have installed with the ACPI HAL, so the virtual devices will be ACPI devices.
The system I'm on at the moment doesn't have every possible hardware interrupt listed, it also goes up to 23 but the virtual IRQs start from 81. There are also some with negative IRQ numbers.
Edit: we also need to allow for legacy hardware that might not exist in a modern system, such as parallel port or FDD, thus will have no hardware IRQ assigned, and the IRQ channels that chain to the next controller (IRQ controllers are 8-channel). So for example IRQ2 normally has no device assigned because it chains to the second controller, and IRQ7 is the default for LPT1, which a modern mainboard usually won't have.
Edited by Platypus, 03 August 2014 - 07:59 PM.