Why is my table displayed at the top of the page?

table is a floating environment. TeX has special rules for placing floats so that they cause minimal disruption of the surrounding text.

You can give LaTeX hints of how to place your floats. For example, to make LaTeX try to place it where you inserted it, specify the option h (for “here”):

\begin{table}[h]
    Table: This table should be below the text, but it is not.  
\end{table} 

Note that this won’t always work because LaTeX still tries to make the text look nice. Your hint is only that: a hint, not a definite command.

You can force LateX to obey your command to place a floating environment HERE by loading the package float and specifying the option H (capital letter!):

\begin{table}[H]
    Table: This table should be below the text, but it is not.  
\end{table} 

But notice that this can lead to an ugly layout.

To quote the FAQ:

Tables and figures have a tendency to surprise, by floating away from where they were specified to appear....

Read the FAQ to learn how to control the placement of your tables.