Both an exponent field of 00000000 and 00000001 encode an exponent of -126, but with 00000000 there is no implicit leading 1 and with 00000001 there is. Consider going from 0x00800000 (smallest normal) to 0x007fffff (biggest denormal). For 0x00800000 the significand (including leading 1) is still 0x00800000 (a nice coincidence), for 0x007fffff is significand is just 0x007fffff. Since the exponent remains the same, these are just as adjacent as they seem (the distance between them is the lowest non-zero denormal).
If the exponent for denormals was -127, the denormals would only fill up to halfway between zero and the lowest normal, leaving a large gap between the highest denormal and the lowest normal.
Normally the number line including denormals looks like this (if you had 2 bits of significand, so 4 numbers per binade): (source)
You can see the denormals and the numbers in the first normalized binade are equally spaced, meaning they have the same exponent. What happens if the denormals had the exponent which their encoding seems to imply?
A gap appears.