If you read the wikipedia article..
"It is not known whether any undiscovered Mersenne primes exist between the 39th (M13,466,917) and the 47th (M43,112,609) on this chart; the ranking is therefore provisional. Primes have also not been discovered in increasing order. For example, the 29th Mersenne prime was discovered after the 30th and the 31st. Similarly, the current record holder was followed by 2 smaller Mersenne primes, first 2 weeks later and then 8 months later."