I found that the jitter of netem is not accurate

I use linksys 1900acs router and I upgrade the router to latest openwrt version. But when I want to test jitter with it. I found it is not accurate. The command is as below:
tc qdisc add dev br-lan root netem delay 100ms 300ms

I want to set 300ms jitter. but I actually have around 120ms jitter.

Does anyone run into same issue?