Starting with a single cell selection, holding down the Ctrl key and selecting
other non-adjacent cells (ie, trying to create a non-contiguous selection) does
not result in a non-contiguous selection being created.  The selection merely
changes from the first-selected cell to the second-selected cell(s).  To
illustrate, click on cell A1.  Hold down the Ctrl key and click in cell B3.  The
selection of B3 is not added to A1.  The selection merely changes to cell B3. 
Similarly, if the second selection is made to be contiguous (ie, adjacent cells
are selected) the selection is removed from the first and changes to the
selection of contiguous cells.  To illustrate, click on cell A1.  Hold down the
Ctrl key and click in cell B3, then drag to cell B5.  The selection of B3:B5 is
not added to A1.  The selection merely changes to cells B3:B5.

Non-contiguous selections *can* be made if starting with a contiguous selection
of more than one cell.  To illustrate, click in cell A1 and drag to A2, thus
selecting A1:A2.  Hold down the Ctrl key and click in cell B3.  The selection of
B3 *is* added to A1:A2.  The selection now consists of A1:A2 plus B3. 
Similarly, adding contiguous selections to contiguous selections also works. 
For example, try starting with A1:A2 and add B3:B5.  The end result is a
selection of A1:A2 plus B3:B5.

A workaround exists which involves *not* starting with a single cell, but
deliberately selecting an unwanted cell, then removing it from the selection
later.  To illustrate, click in cell A1 and drag to A2, thus selecting A1:A2. 
Hold down the Ctrl key and click in cell B3.  The selection of B3 is added to
A1:A2.  With the Crtl key still pressed, click in cell A2.  This removes A2 from
the selection. The selection now consists of A1 plus B3 - something which could
not be achieved if you started with A1 only.  If your intended selection
includes contiguous cells, another workaround is to select them first, then go
back and add single cells to that selection.

The workaround is not particularly difficult, but the intuitive action is to
select a cell, press the Ctrl key, select more cells, etc.  When the first
selection is a single cell and the attempt at adding to the selection fails, you
then have to start again.  This is irritating and counter-productive.  It would
be a good enhancement to have the behaviour work consistently across all methods
of cell selections - ie, make starting with a single cell behave the same as
starting with contiguous cells.