I absolutely abhor shade variants like off-white vs. "dark" white paper or stuff like this here etc. (good luck if you don't own a completely UNC perfectly preserved note 😅). Thankfully banknotes are not my main collection field.
Back to the topic … can you proof that these are regular occurrences or is one note just sun bleached?
P.S. are you using a light source that is lower then 5000K to 5600K? Always suboptimal in threads about color variants and non neutral light gets used without a reference to white balance. If we want to use the pictures in the comments.