d3.max() returning a value 10% larger than actual value

I have an issue with d3.max() returning a number 10% higher than the actual largest number in the set.

I'm using d3js and c3js.

CSV

date,count
2003-01-10,4
2002-01-14,5
2001-01-09,6

JavaScript

values: function(d){
    var tickNumber = 3, //Set tick number
        tickValues = [],
        increment = d3.max(d) / tickNumber;
    for (i = 1; i <= tickNumber; i++){
        tickValues.push(
            numeral(increment * i).format('0')
        );
        console.log(d3.max(d));
        // returns 6.6, not 6
    };
    return tickValues;
}

I've made this work for me by changing this line

increment = d3.max(d) / tickNumber;

To

increment = (d3.max(d)-(d3.max(d)/10)) / tickNumber;

It works but it's a hack and I'm not really satisfied with it. Can you please help me understand why d3.max(d) returns 6.6, not 6?

via Chebli Mohamed