They probably got in there via the various themes and plugins you have installed in the past. Most plugins and some themes store various settings in the wp_options table as part & parcel of their basic functionality. In other words, if they didn't store settings in the wp_options table, they wouldn't work in the way that they're supposed to.
Some plugins offer an uninstall option that deletes anything they have previously saved in wp_options but many don't.
You could remove them manually via phpMyAdmin (which I have done in the past) but make sure that save a full db backup first - just in case you inadvertently delete something you need. However, I seriously doubt that the number of records in this table will be causing you any real performance problems, so you could just ignore them.