Why 3v3 should be connected to the center point of Ethernet termination?
IMO it's a pull-up resistor, TXP and TXN is just a open collector output that ties the line to GND level. The total current is now the current sum from both pull-ups. One goes directly from pull-up to ground, the second goes from pull-up trough a transformer and to ground.
The receive channel is the same, since it is auto-MDIX. Therefore you can use a cross over cable and the TX and RX channels got swapped automaticly.

EDIT:
I said IMO, there is no detailed description about the IC output. Here is the picture of CML output type. Do the 50 ohm pull-ups make sense now?  



EDIT2:
The ethernet transceiver IC differs from manufacturer to manufacturer. It is necessary to use the template or demo board as recommended by the manufacturer. If some manufacturer uses centre tap tied on +Vcc and some else is floating, .... its due to the IC characteristics and can't be swapped or used an universal schematics that will fit all ICs and all magnetics.