I would love to find out why these minor changes came about.
For the dimes, Bowers states in his Barber Silver Coins guide that the 1901 obverse changes lowered the relief, and that Liberty subsequently did not wear away as quickly. The relief for the 1900 Reverse 2 change also seems flatter to me. Other speculation regards die life - there are several letters back and forth between the branch mints and Barber around 1900 regarding die life. Coin stacking differences are noted as a reason for changes to the quarter in 1892 and possibly again in 1900, and Bowers also mentions lowering of the relief for quarters resulting in Liberty not wearing as quickly.
I've been mulling over the reason for the addition of the "thick ribbon" in 1901, as it appears to be the only change to the reverse design, and the 1900 design only lasted a little over a year. I have noted that many 1900-1901 Reverse 2 coins across all mints have a die gouge below the right ribbon, exactly where the thick ribbon later appears. Barber may have been picky and wanted to cover this up on a master die, or perhaps he intended the 1900 change to have the thick ribbon, started to engrave it, and didn't finish? Otherwise, the 1901 obverse changes, and the addition of the thick ribbon, more closely resemble features on the 1891 pattern coin. Perhaps Barber simply thought "I really wanted it to look more like this" and finally got around to it? Idle speculation on my part.