The calculation of how many entries you will get for a rrd4j database is difficult and heavily dependent on your setup!
Since you use the default setting for the numeric entries the archives should be set to:
"AVERAGE,0.5,1,480:AVERAGE,0.5,4,360:AVERAGE,0.5,14,644:AVERAGE,0.5,60,720:AVERAGE,0.5,720,730:AVERAGE,0.5,10080,520");
(Copied out of the code, not from the documentation!)
Which means you will have 480 entries covering the last 480 minutes (archive 1), followed by 360 entries covering the last 4 * 360 minutes i.e. 24 houres (archive 2), followed by 644 entries covering the last 14 * 644 minutes i.e. 9016 minutes (archive 3) …
Your REST API request had no end- or starttime set, so it will make a request for to last 24 houres. From a rrd4j database you will be prompted with all entries of the first archive that covers the requested timeframe, in your case 24 houres. So your return will be from archive 2. This archive does not hold an entriy for each minute, its step size is 4, so you have one value for 4 minutes(calculated by the average of the four minute-wise entries). That correlates with the time-difference of consecutive readings (240000) and the number of entries in that archive (360). Since you just started this database, only those entries with actual readings hold data other then zero (the youngest datapoints are at the END!)
If you want to get the minute-wise entries you would need an REST API request which covers a timeframe of max 480 minutes starting from now!