Because those just starting out in investing generally only know that buying when prices drop is a good way to invest, this is similar to the Lump Sum strategy, which requires waiting for the price to drop before making a purchase, a common practice when first starting out.
You are not buying with lump sum when you are waiting for the price to dip and buy with higher amount, you are only buying at the dip and that's not encouraging. Lump sum is when you use a good apart of money to buy bitcoin once at a go irrespective of the price of bitcoin at that moment.
For instance, you have $200 given to you and you don't have any budget for the money because you are not expecting it. If you buy bitcoin right away with the money, you have lump sum. Buying with lump is a way to front load your bitcoin investment.