This is caused by "dueling auto-sense" circuits.
Whether the link partner is determined to be MDI (a PC, or "normal" connection), or an MDI-X (a hub/switch, or "uplink" connection) is traditionally determined by whether the TX signal is sensed on pair 1,2 or pair 3,6.
Many modern hubs and switches are capable of auto-sensing the attached link partner’s signal, and will accommodate either by internally switching which pair is used by the hub/switch. The sensing is usually accomplished by the simple expedient of alternately offering the TX signal on each of the two pair choices.