When you configure a LLQ like you did, the value you put after "priority" configures a policer that all traffic entering the queue must conform to.
This policer has a CIR, and a committed burst, in your case the CIR is 128K, and the committed burst is 6.4K. Generally if you dont specify a burst, IOS will automatically pick a small one, like you see above.
To visualize it, imagine a token bucket. The max amount of tokens you can store in the bucket is 6400, which is how many the bucket starts with. This bucket replenishes itself at the rate of your CIR, or 128,000 per second. Every time a packet comes in, the size is subtracted from the bucket, therefore if 6500 bytes worth of traffic comes in at once, 100 bytes will be dropped.
That being said, if the traffic you're sending through the priority queue is bursty in nature, you'll get drops due to the fact that you're overrunning your burst.
To allow for a larger burst, instead of putting 'priority 128' under the class, try 'priority 128 128000'. This would effectively double the size of the committed burst, allowing more traffic to conform.
Keep in mind that the priority queue optimizes itself for a steady stream of small packets, such as voice. If you're trying to guarantee data traffic, you're better off using the 'bandwidth' command.
