Try reading the help for raster. When creating a raster from a matrix, the sense of rows and columns isn't what you think it is. You were feeding it a 1241x710 matrix but taking the max and min from the wrong vectors.
Try the following:
> # small version of your test set
> dat1=list()
> dat1$x=seq(302339.6,by=1000,len=71)
> dat1$y=seq(5431470,by=1000,len=124)
> dat1$z=matrix(runif(71*124),71,124)
> str(dat1)
List of 3
$ x: num [1:71] 302340 303340 304340 305340 306340 ...
$ y: num [1:124] 5431470 5432470 5433470 5434470 5435470 ...
$ z: num [1:71, 1:124] 0.765 0.79 0.185 0.461 0.421 ...
> image(dat1,asp=1)
Nice square pixels. Now create your raster:
r <-raster(
dat1$z,
xmn=range(dat1$x)[1], xmx=range(dat1$x)[2],
ymn=range(dat1$y)[1], ymx=range(dat1$y)[2],
crs=CRS("+proj=utm +zone=11 +datum=NAD83")
)
plot(r)
Totally NON-square pixels. And if you look carefully, the matrix is rotated 90 degrees from the image plot. Or transposed or something.
Solution: just create the raster from the x,y,z list:
> r=raster(dat1);plot(r)
Square pixels, same way round as image plot, and resolution is now what you expect:
> r
class : RasterLayer
dimensions : 124, 71, 8804 (nrow, ncol, ncell)
resolution : 1000, 1000 (x, y)
extent : 301839.6, 372839.6, 5430970, 5554970 (xmin, xmax, ymin, ymax)
coord. ref. : NA
data source : in memory
names : layer
values : 7.738103e-05, 0.9995497 (min, max)