Well, either it was followed immediately by two more output lines with shares found in the same enqueued work so the (global) total counter was already increased with +2 more shares, or we have a bug, or it's also possible that either the stratum communication with the pool is messed up or the nanopool themselves messed up and never responded for two of your shares. The first counter is incremented when shares are found, the last two when the pool reports back and we know the accepted/rejected status.
Do you see it continuously, i.e. for your GPU0 you're always missing two shares (unless you've restarted already)?
[2018-11-02 13:43:27] Pool xmr-eu2.nanopool.org share rejected. (GPU0) Error code: -1 - Block expired (438/425/9)
[2018-11-02 13:43:39] Pool xmr-eu2.nanopool.org share accepted. (GPU0) (439/426/9)
[2018-11-02 13:43:52] Stats GPU 0 - cnv8: 2.310kh/s, avg 2.340kh/s, pool 2.387kh/s 426/9
[2018-11-02 13:44:22] Stats GPU 0 - cnv8: 2.310kh/s, avg 2.340kh/s, pool 2.383kh/s 426/9
[2018-11-02 13:44:25] Pool xmr-eu2.nanopool.org received new job. (job_id: 1999)
[2018-11-02 13:44:41] Pool xmr-eu2.nanopool.org share accepted. (GPU0) (440/427/9)
[2018-11-02 13:44:52] Stats GPU 0 - cnv8: 2.310kh/s, avg 2.340kh/s, pool 2.385kh/s 427/9
Even when I restarted, these "unrecorded shares" still appear. The total percentage of them is not more than 2.