sometimes they have the same price, sometimes not. It seems they are not receiving the same information tick by tick.
Not an error, it depends when you request the chart. i.e lets follow two requests, your first request starts on tick 100 (assuming you could actually count them) of the day so your first 17 tick bar is composed of ticks 100-117, now you make your second request and it starts on tick 110, in your second chart the 17 tick bar is composed of ticks 110-127, so the bars will be different.
Unlike time based bars whereby you can align minute bars by the change of the minute hand, there is nothing in existance to align tick bars. They start when you get them.
Richard Carlin