Question about transfering to COCO with Mocov1 and Mocov2 checkpoint

Hi, I get some question when I reproduce the results on COCO.

When I use the Mocov1 pretraining [checkpoint ](https://dl.fbaipublicfiles.com/moco/moco_checkpoints/moco_v1_200ep/moco_v1_200ep_pretrain.pth.tar)released in the repo, I can get the same results reported in the paper as below.
Evaluation results for bbox:
| AP | AP50 | AP75 | APs | APm | APl |
|:------:|:------:|:------:|:------:|:------:|:------:|
| 38.658 | 58.372 | 41.765 | 21.351 | 43.372 | 51.629 |

Evaluation results for segm:
| AP | AP50 | AP75 | APs | APm | APl |
|:------:|:------:|:------:|:------:|:------:|:------:|
| 34.014 | 55.207 | 36.171 | 14.915 | 37.526 | 50.783 |

However, when I change the pretraining [checkpoint](https://dl.fbaipublicfiles.com/moco/moco_checkpoints/moco_v2_200ep/moco_v2_200ep_pretrain.pth.tar) to the Mocov2 one. I get a worse results as below.
Evaluation results for bbox:
| AP | AP50 | AP75 | APs | APm | APl |
|:------:|:------:|:------:|:------:|:------:|:------:|
| 33.921 | 52.401 | 36.497 | 19.247 | 37.598 | 45.513 |

Evaluation results for segm:
| AP | AP50 | AP75 | APs | APm | APl |
|:------:|:------:|:------:|:------:|:------:|:------:|
| 30.113 | 49.341 | 31.897 | 13.713 | 32.557 | 45.618 |

There is a large gap about 4% between the results of Mocov1 and Mocov2. The code I used is in this repo with official setting. Is this a normal phenomenon? Or just I make some mistakes? I am wondering if anyone could kindly give me some advices or explanations. Thanks a lot!