因为你加了 append=TRUE, 看write.table代码
if (!is.null(col.names)) {
if (append)
warning("appending column names to file")
if (quoteC)
col.names <- paste0("\"", gsub("\"", qstring, col.names,
fixed = TRUE), "\"")
writeLines(paste(col.names, collapse = sep), file, sep = eol)
}