Petra,
You get the internationally recognized ?Hmmmmmm? award this morning.
That means when many people see your question they will say to themselves, "Hmmmmmm?"!!
I wish I had a perfect answer for this, but I believe there are actually multiple variables at play here. For those who enjoy learning and/or improving the web, Google MapMaker is a great place to start. This link goes to MapMaker's add a Place page. (those "spots" are called...Places and the icon that is an upside down teardrop - often with a letter in it - is a Marker).
Anyone can edit Google Maps via MapMaker. (All edits are reviewed by others and you cannot review your own edits). One thing you can do is change the shape of something on a map like an area (e.g. a park, stream, etc) or a building. If an editor has used the line drawing feature to change the shape of a feature, that could potentially affect what shows at a given level of zoom. IMO, another thing that could affect what does or does not show at given levels of zoom would be how often that place is searched for, linked to, and/or clicked on. Also, has it been edited by outside editors and then approved? If it has, I would think that site would show more at broader zoom levels than a similar business which had not been edited, especially if there are a lot of places in that given area.
One I tested that proved out: With similar businesses, where one has verified the listing and one has not, the verified listing will stay longer as you zoom further out. (Even with restaurants where the non verified had over 500 reviews and the verified had less than 100 reviews, the verified stayed while the other disappeared).
You also provided me a new mechanism for determining who is or is not an owner verified listing. Thanks!!!
I hope this helped and I urge all to explore Google Map Maker as it really is a COOL TOOL!!
All the Best,
Robert