Bug in DendroPy's birth_death_tree function?

Question

I'm not sure if I'm doing something wrong but I've been trying to run a lineage through time (LTT) function for DendroPy using a birth_death_tree (code snippet below:)

import dendropy

tree = dendropy.model.birthdeath.birth_death_tree(1.0,0.1,max_time = 5.0)

# Returns distance of node furthest from root, i.e., maximum time available on
# tree
total_time = tree.max_distance_from_root()

# Divide time span into 10 steps
step = float(total_time) / 10

# To store tuples of (time, number of lineages)
ltt = []

# Start at first time step
current_time = step
while current_time <= total_time:
    # Get number of lineages at current time
    num_lineages = tree.num_lineages_at(current_time)
    # Store it
    ltt.append( (current_time, num_lineages) )
    # Move to next time step
    current_time += step

# Get the final number of lineages
# Note: may not be the same as the number of tips if the tree has extinct
# tips/taxa; though, if this were the case, we would not be dealing with an
# ultrametric tree.
if current_time < total_time:
    ltt.append( (total_time, tree.num_lineages_at(total_time)) )

# Print results
for t, num_lineages in ltt:
    print("{:12.8f}\t{}".format(t, num_lineages))

I am using the ltt function they have in their documentation and replacing the built in tree with a model tree https://pythonhosted.org/DendroPy/library/birthdeath.html

The output for the number of lineages is giving me a decrease in the number of number of nodes towards the end of the LTT plot. This is unlikely because all the nodes in the model should end at the exact same spot, i.e. a constant. Can someone figure out why this is happening?