The R-square value should be 1. Perhaps the software is giving you the "adjusted" R-square value, which is slightly smaller.

perhaps, or technically, if R-squared is exactly 1 then the adjusted R-squared should also be exactly 1 ... according to their "usual" formula-connection:

https://en.wikipedia.org/wiki/Coefficient_of_determination#Adjusted_R2

http://net-informations.com/ds/psa/adjusted.htm

although some implementations might possess some difference ...

Hi,

The regression line of Y versus X introduces one of the hypotheses that the values of Y depend on those of X, i.e. postulate that the knowledge of the values of X allows to predict the values of Y. It is therefore a forecasting model and the objective is to minimize the forecasting error, i.e. the distance between the observed values Yi and the values Y*i estimated by the relation Y*=mX+c. The residuals will therefore be the distance to the right from the Oy axis.

Also for more information about this subject, I suggest you see the links on the topic.

https://www.linkedin.com/pulse/linear-regression-ashutosh-chaudhary/

https://sphweb.bumc.bu.edu/otlt/mph-modules/bs/bs704_correlation-regression/bs704_correlation-regression_print.html

https://www.youtube.com/watch?v=KeTFzJUG_rA

https://biosearch-cdn.azureedge.net/assetsv6/Calibration-curve-guide.pdf

https://atmos.washington.edu/~robwood/teaching/451/labs/linearregression.html

Best regards