Appraisals - How do they work?

[Josh Green]

Been having a hard time with appraisers. First was a duplex I was under contract on myself. The appraiser used comps over 12 months old (obviously a dramatic 20%+ difference in prices then) which led to a low appraisal (not a ton of comps within 2.5 miles. They used old comps over comps in the 4-5 mile distance and the deal ultimately fell through because of this.

Next, an appraiser valued a 4plex using the income approach (came out to $604k). Rents were estimated way lower. Why did the appraiser use this method when identical 4plexs in the same neighborhood sold just a few weeks prior ($700+k)?

My questions are:

1) why does an appraiser choose income approach vs a comparable sale approach? Is it the type of loan being used to purchase the property (owner occupant vs investment)? Is it anything to do with having fewer than say 5 comps?

2) why would an appraiser pull comps over 10 months old that are within 2 miles from the subject property rather than comps that are 1 month old within 4 miles?