While the PlayStation 4 uses a 8GB unified pool of GDDR5, the Xbox One uses DDR3, which is alleviated by 32MB of "embedded static RAM". This means that bandwidth is more of an issue in the Xbox One. However, Microsoft thought that the console would peak at 102GB/s (as compared to the PS4's 176GB/s), but now sources say that Microsoft has revised the figure, saying 192GB/s is theoretically possible. However, practical estimates put the rate at 133GB/s with alpha transparency blending operations.
Why the goof up? Microsoft took its calculations from a basic figure: 128 bytes per block multipled by the GPU speed of 800Mhz. What the techs didn't know what that the hardware was capable of reading and writing simultaneously.
Generally, the main issue is that this seems to contradict earlier rumors that the CPU of the Xbox One is being downclocked. According to sources, 800MHz remains the clock speed of the graphics component of the processor, and the main CPU is operating at the target 1.6GHz, which keeps parity with the PS4. In essence, the machines are taking different routes to the same goal, with the PS4 having a slight edge in processor speed.