ADB and Fastboot displays different devices

Short answer

Fastboot, (recovery) and your "normal Android" are different systems, each using their own configuration and "device serial".

TL;DR

That's nothing unusual. Most devices¹ use different IDs in normal/fastboot/recovery-mode to indicate the mode they're in. You can cross-check that with the lsusb command on your Linux shell prompt, which reflects this by different device IDs. On top of that, some devices even use different IDs depending on what "normal mode" you've activated. To give an example, here's what my LG P880 presents:

• 1004:61c5 when in charge-only normal mode


• 1004:631c with MTP enabled


• 1004:631e with PTP


• 1004:61f1 using the "LG Software protocol" (to connect with their PC counterpart, which is Windows only)


• 1004:61fe when in tethering mode

Easy to guess: the 1004 remains the same (it's the "manufacturer ID"). But the number behind the colon changes – so the device presents itself with different "product IDs".

However, this should not reflect on the "Serial" – as the device serial in all those 5 cases is taken from the same place (build.prop of the system running in "normal mode"), which is bound to the currently running ROM. Now what happens if you switch to "fastboot mode"? That's not related to the running OS at all (put in "easy terms", you could compare this with a PC's "BIOS" or, more correctly, the "Grub" bootloader). So fastboot doesn't know about the ROM's serial number, and thus uses its own. With recovery-mode it's somehow similar – in our comparison, you chose a different entry in Grub and thus booted a different system.

^{1: in fact, all devices I've tried, so I don't know of any exception – though with the amount of different devices available, I cannot guarantee there is no exception at all}