Accidentally flipped a pair of Ethernet wires. Why does it still work?

Ethernet TX and RX signals (Green and Orange pairs) are transmitted in differential mode aka "balanced":
https://en.wikipedia.org/wiki/Differential_signaling https://en.wikipedia.org/wiki/Ethernet_physical_layer https://en.wikipedia.org/wiki/Ethernet_over_twisted_pair

When using differential signaling the information is carried by the voltage difference (electrical pulses) betweeen conductors. The signal polarity (being which of the two conductors is the "positive" one) is important only in the case the receiver device is not able to auto detect it and automatically invert the received pulses decoding. Most if not all of the modern Ethernet chips are able to auto-sense polatiry. As an example, the following model is able to do automatic polarity detection and correction: http://ww1.microchip.com/downloads/en/devicedoc/8720a.pdf

The so called "polarity fault" has been addressed by the IEEE 802.3 Note that since 100BASE-TX specification, polarity is no more used in receiver.

The following link reports a more detailed analysis:
http://ww1.microchip.com/downloads/en/AppNotes/AN127-UNG.pdf