Each item in haftseen symbolizes a concept:
Sabzeh symbolizes the rebirth and renewal of nature.
Samanu symbolizes fertility and strength.
Senjed symbolizes love and affection.
Serkeh symbolizes patience.
Seeb symbolizes beauty.
Seer symbolizes health.
Sumac symbolizes the sunrise.
Sekkeh symbolizes wealth and prosperity.
Sonbol symbolizes spring's arrival.
Sa-at symbolizes time.
Although only seven of the items above are displayed, some of these seem to be present in most haftseens around the world. These are sabzeh (sprouts), serkeh (vinegar), seeb (apple), seer (garlic), and sekkeh (coins). Based on my personal experience, this may have to do with these items more accessible globally, as it may not be easy to acquire items such as senjed and samanu or even sumac in some areas. Of course, sabzeh may also not be available in most non-Iranian stores, but we can grow it ourselves if we begin the process two weeks before Nowruz. Also, now, with online stores providing most of the items needed for the haftseen table, I would recommend beginning the preparations earlier to allow enough time for shipping and delivery.