As a person ages, there is progressive stiffening of the aorta and its main branches.
As the aorta stiffens with age, it expands less when blood from the heart enters it, causing higher systolic blood pressure. Also because of the stiffening, more of the blood in the aorta and its main branches is forced into other smaller blood vessels when the heart beats, thus, less is present between heartbeats to support the diastolic blood pressure, which becomes lower.
Although your diastolic pressure is on the low side, you should continue to take your blood pressure medications to keep your systolic pressure below 130. Lowering systolic blood pressure reduces the risk for heart attacks and strokes, despite an even lower diastolic blood pressure that might result from the treatment. So, even though your diastolic blood pressure is low, your doctor has taken the right approach by gearing your treatment toward bringing down your systolic blood pressure.
I hope this answers your question.