What do estimated_base and current_base mean in the stats of the node-memwatch

I use the node-memwatch to monitor the memory usage of the node application. The simplified code is as below

@file test.js
var memwatch = require('memwatch');
var util = require('util');

var leak = [];

setInterval(function() {
  leak.push(new Error("leak string"));
}, 1);

memwatch.on('stats', function(stats) {
    console.log('MEM watch: ' + JSON.stringify(stats));
    console.log('Process: ' + util.inspect(process.memoryUsage()));

});

Run 'node test.js', I get the output below.

MEM watch: {"num_full_gc":1,"num_inc_gc":6,"heap_compactions":1,"usage_trend":0,"estimated_base":8979176,"current_base":8979176,"min":0,"max":0}
Process: { rss: 28004352, heapTotal: 19646208, heapUsed: 9303856 }

Does anyone know what do the estimated_base and current_base mean? In the page https://github.com/lloyd/node-memwatch, they are not described detailedly.

Regards,

Jeffrey