DeleteRecord implicitly moves the record pointer to the next record in the record set.  If a call to NextRecord is made after a call to DeleteRecord the record pointer will move twice.  This means that a record will be skipped.  When deleting records in loops every other record will be skipped.

Here is a visual analog.

Record one ß (DeleteRecord is called on this record.)

Record two

Record three

Record four

After the DeleteRecord is called the record pointer is on the second record.

Record twoß (Record pointer is now on record two)

Record three

Record four

Now NextRecord is called.

Record two

Record threeß (Record pointer is now on record three)

Record four

DeleteRecord is called again on record three.

Record two

Record fourß (Record pointer is now on record four)

So, record two has been skipped.

example  by DW:

function DeleteContainerType(&arrType,quoteId) {. . . .ExecuteQuery(ForwardOnly);var record = FirstRecord();while(record){    DeleteRecord();    record = FirstRecord();}. . . .}

Siebel NextRecord And DeleteRecord In Loops Skips Record