My guess is the $35 activated for the current month and you have a $35 balance remaining on your account. This balance should be used when the current month is over.
T-Mobile only "activates" a month at a time. If you put $70 (or $105 or more), it will still say you activated for a month. When that month is over, it will take any funds in your account to "activate" another month.
Manage your T-Mobile account