Since initial grants are larger (roughly 2x) than refresher grants, the equal vesting of the initial grant (old behavior) could cause a significant drop in compensation for new hires after 4 years at the company (the dreaded equity cliff). This would only happen for relatively "flat" performers, as if you're on an upwards trajectory the larger size of later grants would eventually catch up to your initial equity grant.

Let's say the sign on equity was 200 shares, and refreshers were 100. With equal vesting it would look like:

Year 1: 50 shares

Year 2: 50 + 1 x 25 = 75 shares total

Year 3: 50 + 2 x 25 = 100 shares total

Year 4: 50 + 3 x 25 = 125 shares total

Years 5+: 4 x 25 = 100 shares total (20% drop!)

The problem has actually been worse, because grants are actually in dollars and the stock has been growing consistently year over year. Which means earlier grants are worth more in dollars in later years than new grants, despite having the same dollar value at the time of issue. Let's say 20% yoy, which is not far off from average over the last 10 years. We'll work in dollars and say the initial grant was worth $200K at sign-on time and later grants worth $100K.

Year 1: $50K = $50K

Year 2: $50K * 1.2 + $25K = $85K

Year 3: $50K * 1.2^2 + $25K * 1.2 + $25K = $127K

Year 4: $50K * 1.2^3 + $25K * 1.2^2 + $25K * 1.2 + $25K = $177.4K

Years 5+: $25K * 1.2^3 + $25K * 1.2^2 + $25K * 1.2 + $25K = $134.2K (23% drop!)

With the frontloading this becomes:

Year 1: 66 shares

Year 2: 66 + 1 x 25 = 91 shares

Year 3: 44 + 2 x 25 = 94 shares

Year 4: 12 + 3 x 25 = 87 shares (7% drop)

Years 5+: 4 x 25 = 100 shares

With stock appreciation this becomes:

Year 1: $66K

Year 2: $66K * 1.2 + $25K = $104K

Year 3: $44K * 1.2^2 + $25K * 1.2 + $25K = $118.4K

Year 4: $24k * 1.2^3 + $25K * 1.2^2 + $25K * 1.2 + $25K = $132.5K

Years 5+: $25K * 1.2^3 + $25K * 1.2^2 + $25K * 1.2 + $25K = $134.2K

So the dip is entirely smoothed over. Add in natural increases do to career growth and you no longer notice it.