Ethernet PHY and transformer turns ratio
The turns ratio is based on the design of the transceivers in your PHY. The ethernet specification calls for a +/-2.5V signal level over the twisted pair, so if your transceiver outputs e.g. 3.5Vpp, this needs to be 'transformed up' to 5Vpp with a 1:1.414 turns ratio transformer.

If you don't adhere to this, your device may not operate together with other network equipment on the other side of the cable. In practice, Ethernet is extremely forgiving on the signal level front so you may get away with a 1:1 transformer, but cable length and possibly performance will definitely take a hit.