The specs provided are just a bit inconsistent, but both are correct.
The i7-3840QM's standard clockspeed is 2.8GHz. If Turbo Boost is enabled and the temps are not very high, then the CPU can clock itself up to 3.8GHz if only one core is being used. It will be slightly lower if two core are used and a little slower again if all 4 cores are used. This is basically done to keep the CPU from overheating which means it will under-clock itself (throttling); i.e. run slower than 2.8GHz if the temp is too high.
Here's the specs for the i7-3840QMhttp://ark.intel.com/products/70846/Intel-Core-i7-3840QM-Processor-8M-Cache-up-to-3_80-GHz