Array.map() strange behaviour

Consider this code snippet :

 var lvls = [65535,54038,47824,39322,32768,26526,18388,11818,5958],
    halfDiff = 2979;

function anaGetKey(samples){
  var keys = samples.map(function(v) {
    var i;
    var val = v + halfDiff;
    for (i=0; i<lvls.length; i++){
      if (lvls[i] < val){
        console.log("found : "+i+" for "+v+" - "+val);
        break;
      }
    }
    console.log(" => return "+i);
    return i;
  });
  console.log(keys);
}

anaGetKey([6000,12000,54000,65000]);

Why does map return 1 for the 65000 case ?

         |_| espruino.com
 2v09 (c) 2021 G.Williams
>found : 8 for 6000 - 8979
 => return 8
found : 7 for 12000 - 14979
 => return 7
found : 1 for 54000 - 56979
 => return 1
found : 0 for 65000 - 67979
 => return 1
[ 8, 7, 1, 1 ]
> 

It prints 'found : 0', meaning i==0 and the last element of the keys array is 1 ?

What am I missing ?