I am FPGA developer and have not much knowledge about GPUs. I just can make a guess, that no one can guarantee execution time on GPU. There are all little details well defined in FPGA design and result is always available after defined delay.
Though wouldn't it be far more interesting to compare the latencies at these rates, than the throughput? Roughly similar price and throughput, but very different energy efficiency and possibly latency could be rather significant for quite a few applications.
Do you think TitanX has real time capability?