Are you using a P4 with a 533MHz bus?
If so, there is a possibility that your motherboard doesn't support a bus frequency of over 400MHz. If that is so, it's probabaly making that 2.4GHz P4 run at 1.8GHz because normally your P4 would be running at (2.4@133*18), as it's 2.4GHz, 133MHz FSB quad pumped with an 18x multiplier.
If your motherboard is forcing the P4 to run at a 400MHz bus, then it's running at (1.8@100*18), which falls in line with your scenario.
To fix this problem, you might want to look into these possible solutions:
1. Update your BIOS
2. Read the motherboard manual, and check for jumper/dip switch settings to reconfigure
3. Buy a new motherboard which allows you to take advantage of the 533MHz bus.