Seems familiar, doesn't it?
But when I type something like this
Italic was caused by a pair of single *
since it should be pair of double **
Is this normal?
The formatting help page gives these examples:
*This is italicized*, and so is _this_. **This is bold**, and so is __this__. Use ***italics and bold together*** if you ___have to___.
which render as
This is italicized, and so is this.
This is bold, and so is this.
Use italics and bold together if you have to.
So: one asterisk or underscore for italic, two for bold.
The small box in the sidebar only gives some examples of markup, it is not a comprehensive reference.