You don't say where you are located. Those pH numbers are very very low.

Where did you have your tests done? A soil testing lab?

Have you added any soil acidifiers like sulfur, aluminum sulfate, etc.?

Nitrate and ammonium occur naturally in the soil as a result of microbes breaking down organic matter, fixing N from the air, and/or converting nitrate to nitrogen gas. It is quite natural to see some there.

Soil lab (state lab) says ammonium not usually found unless you're fertilized recently. We're in CT and the native soil is extremely acidic (though usually low 4's, not high 3's).

We've never added anything to this soil - it's just barely out of the woods where we cleared for our house (and now the blueberries are growing in since we took the big trees - mostly oak, maple, hickory, pine - out), til we hit ledge and had to move the foundation.

I just looked it up (had a hard time finding anything but references to synthetic fertilizers), apparently ammonium is produced naturally, just in more neutral soils it's converted to nitrate form, but in acidic soils nitrification doesn't occur as easily.

Still don't know why the large difference in pH and nutrient levels in a relatively small area - maybe some difference in underlying minerals (ledge in more acidic part) - flora is pretty much the same (lots of mountain laurel, blueberry, and in the LESS acidic area, some more moss).

I have seen soil pH vary that much over a relatively short distance, it depends on many things.
Do the Blueberries growing in the 3.8 soil blossom?
Do pollinators spend time at the blossoms if they do?
Is the soil too wet, or too dry, at fruit set time?

There are many variables that determine why one set pf plants, growing here, will produce and others, growing over there, will not and we need to look closely at the why to understand that.
For example, why did my Dogwood not produce blossoms this spring whan my neighbors did? Because mine is growing 10 feet fower then his? Why did two of my Apples not blossom this spring while another did so profusely as did my Plumbs and Sand Cherries. What affect does weather have?

High pH sometimes blocks NPK.
High K could cause an imbalance that will block NP.

A soil pH outside about the 6.0 to 7.0 range can make it difficult for plants to uptake and utilize nutrients. Most all plant nutrients are most readily available in the 6.2 to 6.8 range, but some plants, such as Blueberries have evolved growing is soils with lower pHs and struggles growing in soils with "normal" pHs.
Nutrient imbalance can create as many problems as nutrient deficiency. Excess Potash, K, can cause a plant to not be able to properly use Nitrogen, N, and vice versa. An imbalance of Calcium, Ca, and Magnesium, MG, can create problems that give the appearance of a deficiency in either one, because plants need both, in balance, to properly utilize one or the other. so a soil with adequate levels of Ca can still grow plants with Blossom End Rot, of the soil Mg levels are not in balance.

Well, we planted 6 blueberry bushes in the 4.2 soil, mixed a couple shovels of composted manure (5.6 pH) in the holes and also used as mulch/top dressing, no other amendments (manure was High nitrate, Medium ammonium, High P, Medium K and Low Ca, Medium High Mg) so I'm hoping they will do OK, don't really know what Ca requirements are for blueberries but I've never seen on with BER ;-)