Why does ee.Reducer.linearFit produce only one residual per pixel when applied to an ImageCollection?

Question

I am attempting to create a map of R² values illustrating the correlation between radiance and NO2 concentrations in Asia. Each pixel on the map should have its own R².

As the first step of this process, I tried to find the residuals of each pixel by applying ee.Reducer.linearRegression() to a collection of 5 images. It seems like each pixel in the resulting image should have 5 residuals associated with it, since each image would be a separate data point, but there is only one residual given. Why is this the case? How is this single residual representative of all 5 images?

var scale = 10000

var asia = ["Sri Lanka"]//,"Bangladesh","India","Afghanistan","Pakistan","Bhutan","Nepal","Cambodia","Indonesia","Laos","Malaysia","Burma","Philippines","Singapore","Thailand","Vietnam"]

var dates = ["2018","2019","2020","2021","2022","2023"]

var counter = 0

while(counter < asia.length){
  
  var years = []
  
  var countryName = asia[counter]

  // shapefile of national borders, filtered to just our region of interest
  var countries = ee.FeatureCollection("USDOS/LSIB/2017").filter(ee.Filter.eq('COUNTRY_NA', countryName))

  var counter2 = 0

  while(counter2 < dates.length-1){
    
    // grab all the images for one year
    var startDate = dates[counter2]
    var endDate = dates[counter2+1]

    var rad = ee.ImageCollection('NOAA/VIIRS/DNB/MONTHLY_V1/VCMSLCFG')
                    .select('avg_rad')
                    .filterDate(startDate, endDate)
                    .filterBounds(countries)
                    
    // reduce to image
    var radImage = rad.reduce(ee.Reducer.median())

    var no2 = ee.ImageCollection("COPERNICUS/S5P/NRTI/L3_NO2")
                    .select('tropospheric_NO2_column_number_density')     
                    .filterDate(startDate, endDate)
                    .filterBounds(countries)
                    
    // reduce to image
    var no2Image = no2.reduce(ee.Reducer.median())
    
    // resample both images to be the same resolution
    var radProj = radImage.reproject({crs: "EPSG:4326",scale: scale})
    var no2Proj = no2Image.reproject({crs: "EPSG:4326",scale: scale})
    
    // addBands
    var combo = radProj.addBands(no2Proj)
    
    years.push(combo)
    
    counter2 += 1
  }

  print(countryName)

  // create image collection from yearly images
  var allTime = ee.ImageCollection(years)
  
  // perform linear regression
  var linear = allTime.reduce(ee.Reducer.linearRegression(1,1)).clip(countries)
  
  // coefficients irrelevant; select residuals
  var residuals = linear.select("residuals")
  
  print(residuals)
  Map.addLayer(residuals)

  counter += 1
}