I guess a computer wouldn't go with the minimalist aesthetic but it's not inconceivable.
Actually the OP said "I measure it with the sports tracker on my phone (Endomondo)" I don't have a clue what an Endomondo is but I guess if the person used the same device on both bikes and the measurement is based on GPS and not wheel diameter etc. that rules out one variable.
The sun is out, the ice is melting and I actually might take the bike out today for the first time this year. Then I will wonder, as in previous years, why I'm about 3 mph slower on the same bike than I was last fall when I've been pushing myself in spinning classes all winter.