I was asked this question in an interview, I believed this is an Open Question. So any answer would be welcome.
There is more than one exchange where stock can be traded.
For example, there is the New York Stock Exchange and the London Stock Exchange. In fact, if you look at all the exchanges, there is essentially continuous trading 24/7 for many financial instruments (eg US government bonds).
The closing price quoted in papers is usually the price at the close on the NYSE. However, options close after that and so there is after-the-close trading in many stocks with active options, so the price at the close of options trading at CBOE is often used.
The "real" price is always changing. But for the purpose of discussion, using the closing price in NYSE (for NYSE listed stocks) is pretty standard and unlikely to be questioned. Likewise, using Bloomberg's price makes sense.
Using some after-hours or small market quote could lead to differences with commonly accepted numbers - until tomorrow :)