[OSM-newbies] JOSM - Download just changes data?

[Tino Hendricks]

Dear List,

last night I tried to add missing data from my 40 km MTB ride to OSM. After working for 1.5 hours the upload of the changes failed several times with 412 errors.

I tried to drill down the problem by just downloading small areas, change them quickly and upload them back.
To make sure my work is not in vain I download data and directly click on upload changes. 
Why is there an upload necessary, even if I don't even touch one single node??

What am I doing wrong?

Thanks for help

Tino
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