This example may help... i think a lot of people do this:
1) RIS target pre boil volume 8.27gallons, 1.084 SG.
2) boil off 1.5gallon, shrinkage .27, trub loss 1gallon => should leave me with 5.5 into a fermenter, 1/2gallon of yeast/trub loss gives me a full 5gallons
3) post boil OG says 5.5gallons at 1.106, but 84 * 8.27 = 732.48 / 6.5 = 1.112
The math of using 1.084 is fine if you're doing total point calcs... "84 * 8.27 = 694.68". You can take the 694.68 and divide by your ending volume of 6.5 and get the 1.106 value you are planning on ending up with. But if you measure that SG of the 8.27gallons it'll be 86 not 84. That's because your sample at room temperature shrunk...
Someone taking a preboil reading here would assume they were 2 points higher at 1.086 and may choose to boil softer and not loose as much wort due to evaporation to keep a 1.106 OG rather than the now calculated 1.109.
Conversely, someone measuring 1.084 thinks they are "spot on, nailed it" but when they are done they will be 4% off... their final OG will be 1.103 and they'll be left trying to figure out where the other 3 points of gravity went... checking volume markers, was their boil kettle stratified when they took a sample, etc. No, they had 1.084 at 8.27 gallons... but that's really 8 gallons at room temp and they measured 1.084. And 84 * 8 = 672, and 672 divided by final volume of 6.5 = 1.103.
For smaller beers this issue isn't as bad.. because we're talking percentages and only 3-4%. My Amber ale is expected to be 1.042 pre boil in BS (with shrinkage), and displays 1.043 when shrinkage is removed. I really only see an issue when beers are over 1.050 ... because 4% of 50 is when the SG will be drifting 2 or more based on shrinkage.