What you get with the rrddata query is the raw data and nothing else. Now you parse this many values you get back and take the maximum value.
Please don’t mix the maximum from the rrd query with the maximum value over a specific time period.
The rrd min/max/avg are only the aggregated values over a given number of datapoints and a specific time frame.
In your case i would only use the max function to get my rrd data and then only search the maximum value in the result. If you want to build a sum over the last month then it is better to use the avg to fetch the rrd values and then add all values together.
To understand all this a little bit better, the tutorial about RRDs from Alex van den Bogaerdt. The parts about consolidation and resampling are nice to read.
RRDtool - rrdtutorial or http://rrdtool.vandenbogaerdt.nl/