val.log中的PSNR值时高时低？

[One1209]

从我的val.log情况来看，为什么会出现PSNR值时高时低呢？不应该是随着迭代次数的增多，PSNR值稳步提升吗？
以下是我的val.log记录：
找了很久都没发现原因，恳请告知！

24-04-17 15:22:06.665 - INFO: <epoch: 7, iter: 10,000> psnr: 2.1367e+01
24-04-17 16:40:16.885 - INFO: <epoch: 14, iter: 20,000> psnr: 1.7173e+01
24-04-17 17:58:24.560 - INFO: <epoch: 20, iter: 30,000> psnr: 1.8179e+01
24-04-17 19:16:39.499 - INFO: <epoch: 27, iter: 40,000> psnr: 2.1871e+01
24-04-17 20:34:55.209 - INFO: <epoch: 34, iter: 50,000> psnr: 1.8104e+01
24-04-17 21:53:01.576 - INFO: <epoch: 40, iter: 60,000> psnr: 1.5948e+01
24-04-17 23:11:02.831 - INFO: <epoch: 47, iter: 70,000> psnr: 1.6152e+01
24-04-18 00:28:56.065 - INFO: <epoch: 54, iter: 80,000> psnr: 1.4302e+01
24-04-18 01:46:35.971 - INFO: <epoch: 60, iter: 90,000> psnr: 1.9275e+01
24-04-18 03:04:25.122 - INFO: <epoch: 67, iter: 100,000> psnr: 1.9152e+01
24-04-18 04:22:17.947 - INFO: <epoch: 74, iter: 110,000> psnr: 1.6359e+01
24-04-18 05:40:04.073 - INFO: <epoch: 80, iter: 120,000> psnr: 1.8555e+01
24-04-18 06:58:01.322 - INFO: <epoch: 87, iter: 130,000> psnr: 2.0622e+01
24-04-18 08:15:53.929 - INFO: <epoch: 94, iter: 140,000> psnr: 1.6522e+01
24-04-18 09:33:48.300 - INFO: <epoch:100, iter: 150,000> psnr: 1.9023e+01
24-04-18 10:51:47.212 - INFO: <epoch:107, iter: 160,000> psnr: 1.7184e+01
24-04-18 12:09:51.140 - INFO: <epoch:114, iter: 170,000> psnr: 1.9060e+01
24-04-18 13:27:48.133 - INFO: <epoch:120, iter: 180,000> psnr: 2.4105e+01
24-04-18 14:45:56.058 - INFO: <epoch:127, iter: 190,000> psnr: 1.6243e+01
24-04-18 16:03:52.822 - INFO: <epoch:134, iter: 200,000> psnr: 1.7937e+01
24-04-18 17:21:34.655 - INFO: <epoch:140, iter: 210,000> psnr: 2.1026e+01
24-04-18 18:39:27.664 - INFO: <epoch:147, iter: 220,000> psnr: 1.9718e+01
24-04-18 19:57:23.373 - INFO: <epoch:154, iter: 230,000> psnr: 1.7403e+01
24-04-18 21:15:04.094 - INFO: <epoch:160, iter: 240,000> psnr: 2.0071e+01
24-04-18 22:32:57.280 - INFO: <epoch:167, iter: 250,000> psnr: 2.0279e+01
24-04-18 23:50:52.820 - INFO: <epoch:174, iter: 260,000> psnr: 1.7413e+01
24-04-19 01:08:33.645 - INFO: <epoch:180, iter: 270,000> psnr: 1.8354e+01
24-04-19 02:26:23.701 - INFO: <epoch:187, iter: 280,000> psnr: 1.7029e+01
24-04-19 03:44:22.464 - INFO: <epoch:194, iter: 290,000> psnr: 1.7357e+01
24-04-19 05:02:08.237 - INFO: <epoch:200, iter: 300,000> psnr: 1.6554e+01
24-04-19 06:19:56.945 - INFO: <epoch:207, iter: 310,000> psnr: 1.8061e+01
24-04-19 07:37:48.817 - INFO: <epoch:214, iter: 320,000> psnr: 1.8480e+01
24-04-19 08:55:34.300 - INFO: <epoch:220, iter: 330,000> psnr: 1.9598e+01
24-04-19 10:16:14.327 - INFO: <epoch:227, iter: 340,000> psnr: 1.8503e+01
24-04-19 11:38:57.797 - INFO: <epoch:234, iter: 350,000> psnr: 1.7704e+01
24-04-19 13:01:18.061 - INFO: <epoch:240, iter: 360,000> psnr: 1.5860e+01
24-04-19 14:24:21.144 - INFO: <epoch:247, iter: 370,000> psnr: 1.7670e+01