The previous line   
 2. hello   
 3. goodbye

then I get

The previous line
2. hello
3. goodbye

(To avoid too much recrimination, I will confess to a little trickery there. What I wrote is completely honest, but that may not quite be what you see.)

Edit: the above was in response to jbl; Harald's answer, which addresses my petty complaint, appeared while I was typing.

When you type a number followed by a period at the start of a line, the text formatter (Markdown) thinks that you are starting an enumerated list. So the preview shows the start of that list. The formatter works on the principle that the numbers that you enter don't matter. The assumption being that you are more likely to be in the situation where you want to go back and add something, or rearrange the list, than in the position where you really care about the exact numbers that you enter.

Thus if you type

1. First item
2. Second item
3. Third item

then you get

1. First item
2. Second item
3. Third item

if you now remember something that ought to be done between the first and second steps, you can simply add it in:

1. First item
1. Something I just remembered
2. Second item
3. Third item

and you get

1. First item
2. Something I just remembered
3. Second item
4. Third item