There are a number of factors that affect how spot prices are calculated and how they are displayed. Site Y may be using a different data source. If they are using a different data provider their spot prices will almost certainly be slightly different due to variations in calculations, delivery, caching and timing. Site Y may be applying adjustments to the global spot price in order to affect how their site displays pricing data. Site Y may have difference from your site such as speed, latency, delivery, caching, polling intervals and code.