Maybe you are using a different latency.
Example: If you to use in G723.1 a latency of 30mileseconds you will to use about 16Kbps, but if you to use G729 with a latency of 90mileseconds you will to use about 12Kbps.
On the other hand if you to use G729 with a latency of 20mileseconds you will to use 24Kbps.
Maybe in your case you need to adjust the latency to get a good compression.
Although I agree with your comments, I don’t think the term ‘latency’ is the correct one to use.
The times you mention are the intervals between packets being sent. That’s not the same as latency, although because of buffering, it is related to latency.
Latency is the delay introduced by elements in the system (including the CODEC).