Many things can cause the difference, such as:
1. data reliability - just as Robert pointed out;
2. data point(s) used - some use H and L, some use C only;
3. reversal method - looking for continuation of current column first, or look at reversal first;
4. starting point - this only affects the first few columns in the chart.
Solutions? some thoughts here:
1. multiple data sources - I have used Qcharts, eSignal and some other package, they all have "bad tick" from time to time;
2. signal confirmation - other charting style, other box size, timeframe etc.
3. do your own chart - old timers did it, some still doing it today, by hand;
4. risk control - including position sizing.