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I would not expect to see any drops on this interface as it never even reaches 15mb (15%) capacity.
Your expectations might be incorrect. Often percentage of bandwidth capacity measurements are misunderstood.
Let's assume your ingress is 100 Mbps. Let's also assume your measuring over a five minute period. Lastly, assume the ingress transmits at 100% for 1 minute and then stops for 4 minutes. Bandwidth utilization across the 1 minute would be 100% and 0% for the other 4 minutes, but it would be 20% for the 5 minutes.
But if the 100 Mbps was sent at 100% for each 12 seconds, and not sent for each 48 seconds, 5 minute utilization would still be 20% but unlike the prior 1 minute stats of 100% and 0%, each minute would now also be 20%.
So these first two examples show how bandwidth utilization don't reveal what's happening within the measured time period.
Since ingress was same bandwidth as egress, in the above, there would be no queuing.
If ingress is gig, though, suppose gig ingress arrives for 6 seconds and stops for a remaining 4 minutes and 54 seconds. This too would measure as 20% usage across 5 minutes, but since it will take 60 seconds to transmit the same traffic at 100 Mbps, packets will need to be queued. If queuing buffers are insufficient to hold all the packets, some will be dropped.
The above is a long way of saying, if your ingress rate exceeds your egress rate, there can be a need to queue packets, and if queuing is insufficient, packets will be dropped, this even if utilization is "low". Most likely, you have occasional "bursts" if ingress bandwidth exceeds the egress bandwidth.
From your actual stats, the drop rate percentage is so low, you might not need to concern yourself with the few drops you're seeing. If it is a concern, you might be able to reduce the drop rate by increasing egress buffering, but doing so, also increases egress queuing delay.
