What is bit-reservoir
One of the features of the MP3 format is so called bit-reservoir, which can be simply explained as storing information from one frame to the next frame. If an MP3 file or stream is encoded with bit-reservoir feature enabled then the consecutive frames depend on each other. Otherwise they are completely self-contained.
The bit-reservoir feature slightly reduces the overall size of the file.
Bit-reservoir in streaming
In RTP streaming it is desired that consecutive frames are independent. Otherwise, if a packet is lost, the next one or more frames cannot be properly decoded by the receiver, which results, depending on the decoder, either in noise (glitches) or silence.
On Instreamer the bit-reservoir is configurable. Disable ("kept empty") it when streaming RTP.
PC files are usually encoded with bit-reservoir enabled. Therefore if you decide to stream files, make sure that you create the files for streaming without bit-reservoir. You can also re-encode your existing files using e.g. Lame:
lame --nores original_file.mp3 new_file_without_bit_reservoir.mp3