It seems that when you have some events without an END date (which seems to be allowed in Sitefinity by default), the Events widget doesn't always behave correctly. If you select "upcoming events only" setting, then all upcoming events show up (the ones with start + end date, AND the ones that only have start date). However, if you select "Current and Upcoming Events" setting, then you ONLY see events that have both the start AND end date (regardless of whether they are current or in the future).
In other words, if you have future events that only have a start date (no end date),those events do not show up when the Events widget is set to "Current and Upcoming". They DO show up if you only select "upcoming."
I understand that current event cannot be determined without an END date. But for FUTURE events this shouldn't be an issue.