Quote Discrepancy


I used to just log on to Ample(formerly iii.co.uk) for a portfolio of LSE EOD prices.
Having now looked at other software with improved charting capabilities eg esignal and sierra charts/mytrack i notice that very often there is no agreement between all of them regarding EOD bid and offer prices and % gain/loss.
How does one treat/explain these differences?Is there a 'right one'and a wrong one or does one software have later -out of market hours-data than the other?If so which one?Is the % move always based on yesterdays close to today's close or is other data used eg last price.How does one therefore have confidence in a potential chart based on these different data given each would produce different results?
Thanks in advance for your time,
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