@YouTube why do subscribers disappear on YouTube? a message comes that you have a new subscriber after this 3 to 5 subscribers on the account... and such write-off occurs with each new subscription.
Generally, sub count fluctuations are normal. While we remove spam subscribers regularly from your channel, it's likely folks subscribing and unsubscribing as well. Here's more info: https://goo.gl/6B1Eb6 . Hope this helps clarify!