[R] Why the slope and intercept of 95% CI varies at each calculation when using package mcr?

[vod vos]

Hi Everybody,

aa<- c(1,5,10,20,50,40,50,60,70,80,90,100,150,200,250,300,350,400,450,500,550,600,650,700,750,800,850,900,950,1000)
bb<- c(8,16,30,24,39,54,40,68,72,62,122,80,181,259,275,380,320,434,479,587,626,648,738,766,793,851,871,957,1001,960)

library(mcr)

pbreg<- mcreg(aa,bb, method.reg = "PaBa")

pbreg at para

               EST SE       LCI       UCI
Intercept 7.081869 NA -2.824761 20.169193
Slope     1.055312 NA  1.024968  1.096982

but when you calculate again, 

pbreg<- mcreg(aa,bb, method.reg = "PaBa")

pbreg at para

               EST SE       LCI       UCI
Intercept 7.081869 NA -1.834744 20.598912
Slope     1.055312 NA  1.025339  1.095888

pbreg at para show different values of LCI and UCI compared to the first time,  how does this happen?

Thanks.