SP is fictional. In reality, your account has VESTS, and those VESTS remain fixed over time. VESTS are converted into an SP value whenever you look in various tools at the level of delegation, and the reference rate gradually increases the SP value of your VESTS over time.
When you delegate, you actually delegate VESTS, even if the tool you are using says you are delegating SP. We update the levels in our chart as delegations come in, but we do not update every SP value every week. Even if we did, there would still be discrepancies, but they would be smaller.
If you check your MVEST value against our chart, it should be accurate. Some members update their delegations periodically, resetting a 300 that has drifted upward back to 300. Most do not worry about it.