First and foremost, it depends on which price a broker is choosing to display as their default one. Each instrument has three prices:
1) Bid (SELL)
2) Ask (BUY)
3) Trade (Last Trade Price)
If brokers are displaying the same type of price - ask, for example, then they should be equal if they receive their quotes from the primary exchanges of the instrument.
However, some brokers might not choose to work with the primary exchange due to price constraints & rather go with smaller regional exchanges that would have worse prices - a larger difference between bid & ask, also known as the spread.