I just picked up my 15" 2018 MBP today from a retailer (not Apple). I charged it before I started using it (don't know it's original charge level) and it now has 2 cycles. My recollection of posts here on macrumors (may not be 100% reliable) is that a low cycle count > 0 is not unusual.
I also had High Sierra. Using the serial # decoding in this article (be sure to carefully follow the decoding instructions), my MBP was manufactured mid-October 2018, which is 2+ weeks after the introduction of Mojave, so I don't know why I didn't get Mojave. Maybe the SSD's are flashed before final assembly and they can't bother re-doing it - just speculation on my part. No big deal. https://beetstech.com/blog/decode-meaning-behind-apple-serial-number
The 4th character of your serial number should be "X" (manufactured 2nd half 2018).
The 5th character denotes the week of the 2nd half of 2018 it was manufactured. The base of the week is 1, so 1 is the first week of the 2nd half of 2018.
5th char: 1 2 3 4 5 6 7 8 9 C D F G H J K L M N P Q R T V W X Y
week: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
I think yours is normal. When I first powered up my new 15" 2018 MBP (purchased from Adorama) I took screenshots of everything before I wiped the drive clean to reinstall macOS Mojave, I recall mine was charged in the 90% range. I checked a screenshot and it shows the following straight out of the box on first power up: