Mystery over mock poll EVM at counting

A total of five EVMs did not open when taken up for counting and hence were set aside. Officials said it was not a technical glitch but problems with the on/off switch that could be rectified once the counting of votes in other EVMs was over. 

When the malfunctioning EVMs were taken up, four responded immediately and the votes were counted. It was only then that officials discovered that the fifth EVM was used for the mock poll and still had 514 votes which had not been deleted. 

As both candidates (V Ramu and Duraimurugan) concurred this, EVM was set aside and finally, when postal ballots were counted, it resulted in Duraimurugan winning with a paltry 746 vote lead. 

The question is how after many checks, before and after polling, a mock poll EVM was allowed to be part of the regular polling process.