I am involved in the easyDCP development - just to make that clear.
The behaviour you mentioned is normal. It seems that your source material does not carry enough information in order to reach the maximum bit-rate. That is also normal when working with animated content. The JPEG 2000 encoder is smart enough to detect, whether it makes sense to put more data into the images or not.
There are many ways how you can configure JPEG 2000, so the other tool you mentioned seems to follow another approach. From my experience I can tell, that we received this question many times and at the beginning of our development, we compared packages compiled with other solutions and packages compiled with easyDCP. The quality of the easyDCP packages was at least as good as the quality of the other packages. Most of the time we had the feeling, that the quality of the easyDCP-packages was a little bit better. It was not a big difference at all - more tiny details in certain areas of the images we used.
A couple of years ago we made another test that was also very interesting. We made a DCP with easyDCP - as sources we used DPX files. The DCP had an average bit rate of 100 - 120MBit/s. Then we used another DCP-solution to re-compile the existing DCP. That means it decodes the JPEG 2000 files and encodes them again. We ended with a new DCP comprising an average bit rate of 180 MBit/s. Of course, that makes no sense, since JPEG 2000 de- and encoding won't add new information to the images, right?
If you are more interested in this topic you may want to read about VBR (Variable Bit Rate) and CBR (Constant Bit Rate) Coding.