Currently public gardens, private gardens and parks are all rendered with the same icon - a bench under a tree. Private gardens should not be rendered because:
They are not typically of interest to most people
There can be large numbers of them in a small area, slowing down rendering
They can easily obscure actually relevant details.
Here is a typical screenshot from my Galaxy s4. This is not a bug in the data from OSM, as tagging residential gardens is both common and recommended in the wiki. Residential gardens should be tagged as access=private, so I would suggest using the preprocessor to exclude gardens tagged as private from the dataset. Alternatively, a stricter option would be to only include gardens with access=yes or access=permissive.