MT4 Ticks logged not corresponding with 1 min chart bar

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I wanted to log MT4 ticks for external analysis. I used test code in an EA like this (running on a 1M forex chart)

void OnTick()
{
   MqlTick last_tick;
//---
   if(SymbolInfoTick(Symbol(),last_tick))
     {
    Print(last_tick.time,": Bid = ",last_tick.bid,
            " Ask = ",last_tick.ask,"  Volume = ",last_tick.volume);
     }
   else Print("SymbolInfoTick() failed, error = ",GetLastError());
//---        

} 

Looking at the EA log I was expecting to find the OHLC values of a chart bar would correspond to ticks logged within any given minute.

This is not the case - I find OHLC values from bar often have no correspoding entry in tick log for that minute.

I am not really familiar with MT4 workings so maybe someone can explain why this is.

What time reference does MT4 use to collate ticks into a 1min bar for chart (I assume it is server time)

I thought myabe MT4 was missing ticks but the EA does no real processing.

Thanks

John