When you add money to your vault, the system treats each top-up as a separate investment; we call each of these a "tranche." Each tranche operates independently with its own interest rate based on two factors:

1. The amount you're adding

2. The time remaining until your vault matures

Your vault's overall rate, the one displayed on your screen is a weighted average of all your tranches combined. This is why your displayed rate may change when you add more funds.

A Real-World Example

Let's say you're building a 30-day Naira Vault:

Day 1: You invest ₦1,500,000 for 30 days

Interest rate: 20.05%

This is your first tranche

Day 2: You add ₦500,000 (now only 29 days left until maturity)

Interest rate for this tranche: 16.66%

Why lower? Because there are fewer days for this money to earn interest

Your New Effective Rate: The system calculates a weighted average:

(₦1,500,000 × 20.05%) + (₦500,000 × 16.66%) ÷ ₦2,000,000 = 19.2%

Important: Your rate appears to have "dropped" from 20.05% to 19.2%, but you're not losing money. 

Here's why:

Your original ₦1,500,000 is still earning 20.05%, your new ₦500,000 is earning 16.66%. You're earning more total interest because you have more money invested. The app simply recalculates your average rate to give you an accurate picture

Think of it like this: You're not earning less, you're earning more total returns, but the average rate reflects the fact that your newer funds have less time to grow.