Sharepoint - Threshold is exceeded (>5000 elements) but View still working. Why?

As a site collection administrator, the threshold is 20 000 items. If you ask someone that is not an SCA, they will hit the threshold.

A normal user will hit the threshold limit.

List view threshold - greater than 5,000 - Threshold

Specifies the maximum number of list or library items that a database operation, such as a query, can process at the same time outside the daily time window set by the administrator during which queries are unrestricted. When adding or removing a column index, the threshold is 20,000 by default. When deleting a list or folder, the threshold is 100,000 by default. When renaming a folder within the same library, the threshold is 100,000 by default.

List view threshold for auditors and administrators - 20,000 - Threshold

Specifies the maximum number of list or library items that a database operation, such as a query, can process at the same time when they are performed by an auditor or administrator with appropriate permissions. This setting works with Allow Object Model Override.

Software boundaries and limits for SharePoint Server 2016

Update

SharePoint 2016 is a little bit smarter when it comes down to list and libraries performance and creates indexed columns on it's own where needed to improve the overall performance. At 2500 items, SharePoint starts to index columns by itself.

Compare the two lists by going to advanced settings and just below all the columns there should be a link called "Indexed Columns".

SharePoint 2016: List View Threshold

The view on a list is just a query to the Content Database in the back end which stores the data for the list. A SharePoint list can hold millions of items in them. You need to index your list data to help with the threshold limit. List item threshold is a query limit, and not a list limit. Your view will still work, but there is a chance it will timeout if it continues to be queried and you do not do something about the large query.

It will not hurt performance too much to up the threshold limit to 10000 in Central Admin if needed. Make sure to update your default view and your view all items view to index your columns if you are going to pull more than 5000 items in you view query. Here is a link that explains indexing.