On signet of bitcoin core, why didn't the average time between blocks I mined accord with the time I set?

I'm new here. I met some problem when I use signet on bitcoin core in docker container. I used this command to work out the nbits value to set the average mining time which is 60s.

python3.8 miner --cli="bitcoin-cli" calibrate --grind-cmd="bitcoin-util grind" --seconds=60

But then I use this command to mine blocks, the average mining time is not what I set.

python3.8 miner --cli=”bitcoin-cli -datadir=/data/bitcoin/signet-custom” generate --address=$Address --grind-cmd=”bitcoin-util grind” --nbits=1d30783d --ongoing

I got this:

2022-07-11 18:09:20 INFO Mined block at height 25; next in 4m31s (mine)
2022-07-11 18:13:56 INFO Mined block at height 26; next in 2m40s (mine)
2022-07-11 18:16:37 INFO Mined block at height 27; next in 2m44s (mine)
2022-07-11 18:16:37 INFO Mined block at height 27; next in 2m44s (mine)
2022-07-11 18:19:26 INFO Mined block at height 28; next in 2m40s (mine)
2022-07-11 18:22:08 INFO Mined block at height 29; next in 2m43s (mine)
2022-07-11 18:24:52 INFO Mined block at height 30; next in 2m44s (mine)
2022-07-11 18:27:43 INFO Mined block at height 31; next in 2m38s (mine)
2022-07-11 18:30:22 INFO Mined block at height 32; next in 2m44s (mine)
2022-07-11 18:33:07 INFO Mined block at height 33; next in 2m44s (mine)
2022-07-11 18:35:54 INFO Mined block at height 34; next in 2m42s (mine)
2022-07-11 18:38:41 INFO Mined block at height 35; next in 2m40s (mine)
2022-07-11 18:41:25 INFO Mined block at height 36; next in 2m41s (mine)
2022-07-11 18:44:10 INFO Mined block at height 37; next in 2m41s (mine)
2022-07-11 18:46:53 INFO Mined block at height 38; next in 2m43s (mine)

I'm very confused. Maybe this question is stupid, but I will appreciate it if anybody can help me.

By the way, I wonder if I can use RPC command to control my signet remotely. Thanks a lot!

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