The range of the analog inputs is 0-4.096V, the range of the analog outputs is 0-5V:
http://blog.bela.io/2016/11/07/bela-analogue-vs-audio/
In case you found a mismatch somewhere, do let us know so we can fix it.
Truth is the maximum output voltage of the analog outs is the voltage of your 5V power supply. Occasionally USB power supplies and USB ports do not provide a full 5V, especially when providing a non-trivial amount of current - as it is the case with the BBB.
So I would recommend you check what voltage you get at the 5V pin on P9_7: that should be just a couple of hundreds volts above the level of the analog out when you output a 1
.
The analog voltage you obtain when sending a 0.5
value to the output should be close to the nominal 2.5V regardless of the above measurement. This seems to be the case for you from the measurements you report.
On the Bela I have here, powered from a 2016 MacBook Pro's USB port, I read 4.98V at P9_7, about 4.96V at the analog outputs when sending a 1
and 2.48V when sending 0.5
.