In the second query, FIRSTROW is set to 2. Running these queries separately will return different counts, but running them in an UNION ALL query, the both values will be the same.
SELECT
count(*)
FROM
OPENROWSET(
BULK 'https://teststorage.dfs.core.windows.net/data/test.csv',
FORMAT = 'CSV',
PARSER_VERSION='2.0',
FIRSTROW = 1,
FIELDTERMINATOR = ';' ) as res
UNION ALL
SELECT
count(*)
FROM
OPENROWSET(
BULK 'https://teststorage.dfs.core.windows.net/data/test.csv',
FORMAT = 'CSV',
PARSER_VERSION='2.0',
FIRSTROW = 2,
FIELDTERMINATOR = ';') as res

