Why-Nasdaq-100-in-WealthData-has-296-symbols-and-not-100

Those symbols represent the companies that were in the Nasdaq 100 at some point in time, but were removed from the index. It's a dynamic index, and when you backtest using it (and the other WealthData indices) you get a true to life result because it correctly applies the stocks that were in and out of the index at the point in times of your simulated buys.

Adding to the website FAQ, this is becoming a popular question.

There many interesting and useful features in WL7, this is one of them. It’s to prevent survivorship bias.

Can you explain the naming convention of the symbols having a date and a letter suffix?

Here it is, under "Delisted Symbols":
https://www.wealth-data.com/faq