There maybe some other non soluble material in the various salts, some of the sulfates are derived from a wide range of materials and can be less pure than say KNO3 or Kh2PO4 etc.
A little heat or hot water can often help.
Same with Trace mixes.
The other trick is adding about 1 ml per 10 mls of water with 5% distilled Vinegar, sake good and wait a few min, then add the salts. This removes the KH(a lot of or all).
I have a lot of Ca(NO3)2 and Mg(NO3)2 from dosing marine planted tanks and gave it a try, but did not find any added benefits to Tobi's mix. If you dose lean and then dose richer, then this can lead to various differences. But if you dose rich and simply change the type of source of the Ca, NO3, K+ etc, this should not matter.
This is more likely what occurs.
Because once you mix all these ions together, they completely disassociate in water. So as far what is present......... it will be the same. The all in one Hoagland's solution and tougher chealtors for Fe, tend to use EDDHA to address PO4 and Fe precipitation, but that's bloody red and more suited for marine system's and pH ranges.
Not sure if DTPA and some vinegar would make a decent all in one. Paul K and Neil Frank used it I think, but that was 15 years ago. I know Neil liked using Ca(NO3) since it was easier for him and his water had little Ca++.
But so does mine today, I have no issues.