R数据帧中的布尔加法会产生布尔值而不是整值

如果我try 通过一步添加3个布尔函数来在R rame中创建新列，则会导致布尔值而不是integer值.如果我使用中间步骤首先为3个布尔表达创建列，我可以将它们相加并获得一个整元.我不明白为什么两组代码会产生不同的结果.

#The input is a dataframe with 3 variables that are sometimes missing
#and sometimes not.
subjid <- c(101,102,103,104,105,106,107,108)
var1 <- c(1,2,3,4,NaN,NaN,NaN,NaN)
var2 <- c(1,2,NaN,NaN,5,6,NaN,NaN)
var3 <- c(1,NaN,3,NaN,5,NaN,7,NaN)
df <- data.frame(subjid, var1, var2, var3)
df

subjid var1 var2 var3
1    101    1    1    1
2    102    2    2  NaN
3    103    3  NaN    3
4    104    4  NaN  NaN
5    105  NaN    5    5
6    106  NaN    6  NaN
7    107  NaN  NaN    7
8    108  NaN  NaN  NaN

#This code was intended to count how many of the 3 variables were nonmissing
#But it produces an unexpected result
df$nonmissing_count_a <- !is.na(df$var1) + !is.na(df$var2) + !is.na(df$var3)
table(df$nonmissing_count_a)

FALSE  TRUE
5     3

#This code is intended to obtain the same count of nonmissing variables
#And it works as expected
df$var1_nonmissing <- !is.na(df$var1)
df$var2_nonmissing <- !is.na(df$var2)
df$var3_nonmissing <- !is.na(df$var3)
df$nonmissing_count_b <- df$var1_nonmissing + df$var2_nonmissing + df$var3_nonmissing
table(df$nonmissing_count_b)

0 1 2 3
1 3 3 1